USB WiFi driver for AP mode

Hi,

we are looking for well supported USB WiFi chips/modules with the following 
requirements

 * Be able to support AP mode
 * Support at least 20 concurrent clients (stations)
 * Support multiple SSIDs
 * (preferred, optional: Support 2x2 802.11gn)

We did a survey through the various wiki pages and online information, but 
didn't find anything yet matching those requirements (and only found out 
limitations after trying ...). Any pointers are appreciated. :)

Thanks!
      Simon