In my house the coverage of the Xiaomi gateway/hub is better comparing to a cc2531 with zigbee2mqtt. That’s no surprise as the Xiaomi is a bigger box with probably a much better antenna inside then the tiny pcb antenna on the cc2531. But it should be no problem if there are mains connected zigbee devices that can act as zigbee router and/or you use cc2531 devices as routers as explained before.
The count of devices is of which devices connect to the concentrator or to the router. When you have around 16 devices (newer zigbee2mqtt image than mine limits to 20, see below) connected to the concentrator it probably will search for a router in the zigbee mesh network. With concentrator and routers the total would be 50 as this has something to do with connections tables and available memory space in the cc2531 as is mentioned in discussion threads in the zigbee2mqtt git pages.
See also here:
The CC2531 USB sniffer coordinator firmware has a limit of 20 direct children. This means 1 coordinator + 20 end devices and 0 routers.
However by adding routers to your network you can overcome the limit of 20 devices. This means that, e.g. a network of 1 coordinator, 4 routers and 50 end devices is possible.
The number of childs that a router support differs per device! Therefore, the increased size of a network by adding a router is arbitrary.