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.