Eric,
As far as I am aware - theoritically no limit, but there has to be some limits right!
Your WCS is limited to the amount of AP's it manages! by the license keys
The question should be - how many WLAN's are configurable on a single AP. This of course depends on the AP:-
The 10x0 AP's can support 16 concurrent WLAN's
All other Cisco AP's have a limit of 8 concurrent WLAN's
HTH.