Based on your access point model, you can have configured 8 or 16 simultaneous SSIDs, but as each WLAN/SSID needs separated probe responses, and beaconing, the RF pollution increases as more SSIDs are added. The results are that some smaller wireless stations like PDA, WiFi Phones and barcode scanners cannot cope with a high number of basic SSID (BSSID) information. This results in lockups, reloads or association failures. Also the more SSIDs, the more beaconing needed, so less RF space is available for real data transmits.
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a0080810880.shtml