Hello,
When this AP is controlled by the 850 controller, the client count per radio is by default limited to 64. You can change the limit, the possible values are between 1 and 124. So from configuration perspective 124 is the maximal client count per radio.
However this doesnt mean that the AP can adequately service 124 clients per radio. In certain scenarios even 64 can be too much.
The max number of clients that can be reliably serviced by an AP depends on many factors and no general recommendations can be given. Usually it is determined after a requirements analysis, wireless site survey, RF measurement and performance testing.
For rough estimate you should keep in mind that the data-rate of a radio is not the real throughput. The throughput can be max 50% of the advertised data rate and it also depends on many factors. WLAN is shared medium and this throughput should be devided by the number of clients. This means the more clients per radio you have the lower the per-client bandwidth.