Cisco says that no more than seven concurrent calls use G.711 or eight concurrent calls use G.729. Beyond that number of calls, when excessive background data is present, the voice quality of all calls becomes unacceptable.
It also depends on the wireless PTP link. By this I mean, what is the throughput you have between the two sites. I have seen more than 20 calls on a gigabit wireless link. Again, your bridge must be able to support QoS and you will need QoS running on both ends of the infrastructure. It is hard to estimate that not knowing what bridges you will be using, how far the link is and what the link speed will end up at.