The amount of Zigbee 1.2 devices should not be a problem if have enough Zigbee Router devices, but could get worse latency performance if there are too many jumps to a device. A possible workaround for that is to enable ”source routing” but then need to be sure to have great Zigbee Router devices and there is a risk of the network becoming unstable if a Zigbee Router goes down or is not fully up to task, which is why ”source routing” is not generally recommend (and has to be enabled manually).
Regardless, I would also be sure to utilize Zigbee Groups as much as possible for direct control.