Hello,
You can have without any problems 7 or 8 or 10 non-root bridges associated with a root-bridge with omni antenna. The problems is the available bandwidth. All the non-root bridges will share the same 11Mbps.
So on the bandwith side, there is not a general response, it depends on a the usage you plan and the applications you are running ...
For a reminder in a classic corporate desktop configuration, the Cisco design recommandation is to have arround 20 clients for an AP.
If you have 8 non-root bridge with 2 "classic" desktop clients max per non-root bridge, you are not far from this design.
But don't forget that you have 11Mbps shared, and the response depend on your applications and the throughput needed.
Hope this helps,
Vincent