i run 3 WoW on an imac 1gig 2.0Ghz.. im guessing you can run about the same maybe more, but you will have to try it out and figure out how many you want to run cuz the playability is up to you...

to play more than one WoW you have to go to your applications and just click the World of Warcraft folder then [command+C] [command+V] to copy+paste rename it so you have a new folder (ie:WoW2)

just go into the folders and drag the World of Warcraft icon onto your bar/desktop
you need to do this for each copy of WoW
and voila!