The maximum number of devices that can be connected to Zigbee Router device depends on the limit set by its firmware developer. So some Zigbee Router devices can only route 5 devices while others can handle 50, (though for most is closer to 5). For reference see TubeZB’s Zigbee Router/Repeater → Zigbee Router/Repeater – TubesZB (while other commercial dedicated Zigbee Router products like IKEA Trådfri Signal Repeater are known route 10-20 devices).
Tip is to buy a few dedicated Zigbee Router devices or DIY your own by flashing Zigbee USB adapters with Zigbee Router firmware then powering them via USV chargers. Check out → Zigbee networks: how to avoid interference and get the best range and coverage