You would limit users per radio using the Max Associations parameter in the SSID profile http://www.arubanetworks.com/techdocs/ArubaOS_64_Web_Help/Web_Help_Index.htm#ArubaFrameStyles/VirtualAPs/SSID_Profiles.htm By default this is 64.
Clientmatch is enabled by default and will start load balancing devices at the "cm-lb-client-thresh" (clientmatch load balancing) parameter in the ARM profile: http://www.arubanetworks.com/techdocs/ArubaOS_64_Web_Help/Web_Help_Index.htm#ArubaFrameStyles/ARM/ARM_Profiles.htm%3FTocPath%3DAdaptive%20Radio%20Management%7C_____4
Client Match Enabled
Client Match report interval (sec) 30
Allows Client Match to automatically clear unsteerable clients after ageout Enabled
Client Match Unsteerable Client Ageout Interval 2 Days 0 Hours
Client Match Band Steer G Band Max Signal (-dBm) 45
Client Match Band Steer A Band Min Signal (-dBm) 75
Client Match Sticky Client Check Interval (sec) 3
Client Match Sticky client check SNR (dB) 18
Client Match SNR threshold(dB) 5
Client Match Sticky Min Signal 65
Client Match Restriction timeout (sec) 10
Client Match Load Balancing threshold (%) 20
Client Match IOS Steer Backoff interval (sec) 300
Client Match VBR Stale Entry Age (sec) 120
Client Match Max steer failures 2
Client Match Load Balancing client threshold 30
Client Match Load Balancing SNR threshold (dB) 30
Client Match Load Balancing signal delta bound (dB) 5
Client Match 11v BSS Transition Management Enabled
The bandwidth that a user is restricted is configured in the user role.