The short answer

It's unlikely that you will run into any limits.

The long answer

Zigbee has 2 device types:

• routers, such as smart plugs or bulbs
• end-devices, such as battery-powered sensors

If you have enough routers, there’s no theoretical limit:

• If you have 0 routers, you can have a max of 32 end-devices (connected directly to the coordinator). Running the Zigbee network without extra router devices is not recommended.
• If you have a few routers spread throughout all rooms, it's unlikely you'll encounter a limit. How many children can connect to a router depends on the router’s firmware but each router can usually accept at least five end devices.
• There is no practical limit to the number of routers on a network.