That's a great word, "theoretical"... Marketing says that theoretically, 500 Open Auth clients can all *associate* to a dual-band AP at once.
But... if they all tried to pass traffic, they wouldn't have much bandwidth between them to share.
Realistically, as most clients since 2005 have WPA2 capabilities, you can expect to have 200+ AP clients on both bands using the network.
There won't be as many WPA/TKIP only clients to take up slots.
How many guests are you expecting, and how many APs do you have to manage, on what version of ZD firmware?
(you want to manage your APs with a ZD for seamless roaming)