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Why the answer depends on the hub
“Zigbee hub” describes a type of device, not a shared capacity standard. Manufacturers publish limits using different categories and conditions. One specification may count battery-powered devices connected directly to the hub; another may describe child devices across a network that uses repeaters; a lighting system may publish separate limits for lights and accessories. Those figures are not interchangeable.
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Door and window sensors are commonly battery-powered. In its Hub M1S Gen 2 FAQ, Aqara classifies battery-powered products as end devices and mains-powered Aqara or Mijia products as routers. That distinction matters when a hub’s limit is specifically for end devices. A router-assisted maximum may be higher, but it is not necessarily the number of sensors that can connect directly to the hub.
Capacity is also different from compatibility and dependable operation. A hub might advertise a large accessory or child-device count, but that does not establish that every Zigbee sensor model will pair with it, or that every installation will work reliably at the maximum. Check the hub’s supported-device information and consider the actual layout of the building and network.
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Published limits from three hub systems
The figures below are manufacturer-published, model-specific examples. Read the category and conditions alongside each number rather than treating the largest figure as a universal sensor allowance.
| Hub | Published capacity | What it means for door sensors |
|---|---|---|
| Aqara Hub M1S Gen 2 | Up to 32 Zigbee end devices initially. Aqara says adding Zigbee routers can expand support to up to 128 Zigbee accessories, with each router extending support for 16 end devices. | The initial end-device figure is the closest stated category for battery-powered sensors. The larger accessory figure is conditional on routers and should not be read as 128 sensors directly connected to the hub. |
| Aqara Hub M2 | Up to 128 Aqara Zigbee child devices; Aqara says repeater devices, such as a Smart Plug or a Wall Switch with Neutral, are required. | This is an M2-specific child-device claim with a repeater condition. It is not the same specification as the M1S Gen 2 figures. |
| Philips Hue Bridge | Up to 50 lights, with a hard limit of 63, and 12 accessories. | Hue lists sensors in the accessory category. The light count is not the sensor allowance. |
| Philips Hue Bridge Pro | 150+ lights, with a hard limit of 186, and 50+ accessories. | The accessory figure is the relevant published category for sensors; the larger light limit does not mean that many sensors can be added. |
These are the figures listed on Aqara’s Hub M1S Gen 2 FAQ and Hub M2 product page, and Philips Hue’s FAQ; the pages were accessed in 2026. Manufacturers can change product specifications, so check the current page for the exact model and region before planning an installation.
How to interpret a hub’s capacity number
Direct end devices
A direct end-device limit describes a category of devices that communicate with the hub without acting as network routers. Aqara’s M1S Gen 2 FAQ identifies battery-powered products as end devices and gives an initial maximum of 32 Zigbee end devices. If your sensors are battery-powered, this is a more relevant starting point than a headline number for accessories or lights.
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Child devices
A child-device count is the term used on Aqara’s M2 page for its stated maximum of 128 Aqara Zigbee child devices. The page also says repeater devices are required. Do not silently convert that maximum into a claim that the M2 supports 128 sensors without repeaters, or apply the M1S Gen 2’s direct-device wording to the M2.
Accessories and lights
Accessory and light counts describe manufacturer-defined categories. Hue publishes separate numbers for lights and accessories, with sensors included in the accessory category. If you want to estimate how many Hue sensors a Bridge can handle, use the published accessory limit—not the much larger light figure. Likewise, an “accessories” figure from one product line does not automatically describe another hub’s sensor capacity.
Total devices and router-assisted capacity
A headline total may include multiple device roles, or assume that mains-powered routers extend network capacity. For the M1S Gen 2, Aqara says each Zigbee router can extend support by 16 end devices and that routers can expand the system to as many as 128 Zigbee accessories. Those are linked but distinct statements: the 16-device increment is expressed as end devices per router, while the overall maximum is expressed as accessories. Keep the manufacturer’s terms intact when comparing it with another model.
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Do you need a Zigbee repeater to add more sensors?
It depends on the hub and the capacity you need. A repeater is not a universal Zigbee requirement for every installation, but some hub makers require or specify routers for higher published counts. Aqara’s M1S Gen 2 FAQ describes adding Zigbee routers to expand beyond its initial 32-end-device allowance. Aqara’s M2 page says repeaters are required for its stated 128-child-device maximum.
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Aqara names a Smart Plug as an example router/repeater in the M1S Gen 2 FAQ. That is a hub-specific example, not a guarantee that every smart plug, every regional version of a product, or every mains-powered Zigbee device will serve as a compatible router. Verify the exact product and hub documentation before buying or relying on one.
Adding repeaters does not remove the need to check the sensor limit. A repeater can alter the network path and the capacity condition described by a manufacturer, but the published count still depends on that hub’s model-specific rules. Nor does the presence of a repeater prove that a network at its maximum will have reliable coverage in every building.
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How to find the maximum for your own setup
- Identify the exact hub model. Record its model name and generation, not just the brand or the phrase “Zigbee hub.” If the manufacturer distinguishes regional versions, note the region as well.
- Identify the exact sensor model. Check the full model name or SKU and whether the hub’s supported-device information includes it. A general Zigbee label does not establish compatibility with a particular hub.
- Find the current manufacturer specification. Use the hub maker’s documentation for that model. Look for the exact device-count wording and any firmware, repeater, or compatibility conditions.
- Determine what is being counted. Is the number for direct end devices, child devices, accessories, lights, or all devices? Use the category that matches your sensors; do not substitute a limit from another category.
- Check whether repeaters are part of the stated maximum. Note whether they are optional, required, or described as adding a specified number of devices. Confirm that the proposed repeater is supported for the exact hub and regional product version.
- Count the devices already on the hub. The published maximum concerns a hub’s stated capacity, not an unused allowance guaranteed to be available for sensors alone. Check whether the manufacturer’s count includes other accessories or device types before estimating remaining room.
- Plan for the physical installation. Consider where the hub, sensors, and any routers will sit in the actual building. The specifications cited here do not provide a universal performance guarantee at the maximum, so do not treat the count alone as proof that every sensor will communicate dependably.
- Add devices in stages and confirm operation. Pair sensors according to the hub maker’s instructions, then verify that each appears in the system and reports as expected from its intended location. If a sensor fails to pair or communicate, check support status, the model’s instructions, network placement, and the hub’s count before assuming the capacity number is wrong.
How the published examples compare—and what they do not prove
The examples show why model-level wording matters. Aqara gives both an initial end-device figure and router-related expansion information for the M1S Gen 2. Its M2 page instead states a child-device maximum and a repeater condition. Hue separates lights from accessories, so sensor capacity belongs to the accessory category. No direct apples-to-apples ranking follows from those figures: the categories and conditions differ.
For Hue Bridge, the standard model’s listed capacity is 12 accessories, while Hue Bridge Pro is listed at 50+ accessories. Those are the stated accessory counts, not a guaranteed number of any particular third-party sensor. The Hue figures also include separate light capacity information; the light maximum should not be used to infer sensor count.
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A historical SmartThings Community post by veeceeoh (Keith G), dated February 6, 2020, described HubV2 as supporting 32 directly connected end devices, with routers not counted against that direct limit. It is a community post, not a current official SmartThings support specification. It should not be used to determine a present-day HubV2 limit or generalized to other SmartThings hubs.
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Common mistakes when estimating sensor capacity
- Using the biggest number on the product page. A maximum for lights or total accessories may not be the sensor limit. Match the count category to the device you intend to add.
- Assuming all Zigbee devices are supported. The radio protocol alone does not establish that a specific sensor model is on a hub’s supported-device list.
- Mixing hub generations. The M1S Gen 2 and M2 have different published wording and conditions. One model’s figures do not fill gaps in another model’s documentation.
- Counting a repeater as if it were a sensor allowance. Router-assisted expansion is a condition of some specifications, not proof that any number of sensors can be added. Use the actual stated maximum and conditions.
- Assuming the maximum guarantees performance. A published device count is not a building-specific coverage promise. Radio layout and device compatibility remain separate checks.
- Relying on an old forum answer as current policy. Historical community guidance can be useful context, but it is not a substitute for the current manufacturer specification for the exact hub.
What to do if the hub will not add another sensor
When pairing fails, a capacity limit is only one possible explanation. Work through the checks in this order so that you do not buy extra hardware based on the wrong diagnosis:
- Confirm the sensor’s exact model is supported by the hub, and follow the pairing procedure specified for that model.
- Check the hub’s current documentation for the applicable count category and whether existing lights, accessories, or other devices share that allowance.
- If the manufacturer requires routers for the higher capacity, confirm that compatible routers are present and recognized; do not assume an arbitrary mains-powered product qualifies.
- Check the sensor and hub locations against the intended network layout. A sensor that is beyond dependable radio coverage may fail even if the hub has not reached its stated maximum.
- If the stated capacity appears not to have been reached and the sensor is listed as supported, consult the hub maker’s current troubleshooting guidance for that model and firmware rather than applying a limit from another product generation.
The practical answer
For an exact number, begin with the hub model. The published examples range from Hue’s accessory-specific counts to Aqara’s model-specific end-device and child-device figures, and each uses different terms and conditions. Once you know the hub and sensor models, use the manufacturer’s current supported-device information, identify whether repeaters are required, and plan for actual coverage. Without the hub model, there is no defensible single number for how many Zigbee door sensors one hub can support.
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