Here some data from a presentation...
With 6 SSIDs on this 3APs in one area and a min basic rate of 1Mb (2.4GHz band) you get a channel util of ~26%.
That would mean that 1/4 of the bandwidth is uses for managment frames (beacon frames).
Less SSIDs and a higher min. basic rate could reduce that a lot so you've more airtime/bandwidth for real user data.
You'd have up to 8 SSIDs per radio = 16 SSIDs per AP which isn't a problem for the AP but result in more managment frames = less airtime/bandwidth for user data.
As Craig mentioned 3 is a recommendation...
i.e.
#1 = SSID with 802.1X authentication for company clients
#2 = SSID with WPA-PSK for company clients that don't support 802.1X
#3 = SSID for guests
-Ron