FYI, I further answered and explained the actual device limitations in more details as reply this other thread here which asked the same question → Switched from Sonoff to Skyconnect, burt still limited to 20 devices?
If your Zigbee Router devices are “in-wall” switches then you be aware that, epecially compares to lower frequency technologies like Z-Wave, Zigbee’s 2.4GHz low-energy and short-burst wireless technology has very bad radio propagation (meaning bad penetration of building materials) so if your junction boxes are made of metal or your walls are made of dense building materials then the radio signals will not go far though those walls/floors/cealing, so you will need to add additional Zigbee Router devices to further expand the reach (range and cover) of you Zigbee network mesh. When in doubt, add more Zigbee Router devices! 
Regardless, if you read and do your best to try to follow all the tips in this guide before troubleshooting any deeper then you should be fine → Zigbee networks: how to guide for avoiding interference + optimize using Zigbee Router devices (repeaters/extenders) to get a stable mesh network with best possible range and coverage
PS: By the way, the ZHA integration documentation does also cover some of that and more but in less detail as those docs not meant to be a step-by-step guide → Zigbee Home Automation - Home Assistant