One of my experiences with my USB stick was, that it supports only 8 paired devices as default.
I have a qivicon usb stick flashed with some 5year old zigbee stack. https://community.home-assistant.io/t/eu-usb-sticks-for-the-new-zigbee-component/16718/10
You can easily increase the amount of supported childs with some lines in the bellows code I have now ~20 devices (bulbs, plugs, sensors) in my home.
I assume this restrictions depends on the used zigbee usb stick/device and the installed stack software.
These 4 lines made the difference for me:
yield from self._cfg(c.CONFIG_ADDRESS_TABLE_SIZE, 16 )
yield from self._cfg(c.CONFIG_NEIGHBOR_TABLE_SIZE, 16 )
yield from self._cfg(c.CONFIG_SOURCE_ROUTE_TABLE_SIZE, 32 )
yield from self._cfg(c.CONFIG_MAX_END_DEVICE_CHILDREN, 32 )
have a look in my bellows fork for details