Use as few SSIDs as you can to leave ample room for data. The more SSIDs, the more management and Beacon traffic. Also, look at dynamic vlan assignments. That can really help to reduce the number of SSIDs. 3 is a good number - Corporate, Guest and Legacy.
There are several calculators that show the amount of overhead associated with the number of SSIDs to number of APs per channel and compares data rates and through-puts. You will be amazed at how little room there is for data once you start adding more SSIDs.
Keep your SSIDs configurations simple and the numbers low. Your users will be thankful.