You're referring to the RRM Profile Threshold for Traps setting, right? This does nothing to restrict the number of clients allowed on an access point. This number simply determines when an access point sends an SNMP alert to the controller. By default, once the 13th client hits an access point, it alerts the controller.

Again, this does not affect the functionality of the access point or controller, or even the RRM algorithms. As such, nothing happens to the 13th client. You can read about it in this document:

http://www.cisco.com/en/US/docs/wireless/controller/4.2/configuration/guide/c42rrm.html