2 Max without doing any odd tricks.
There's been some indication it might be possible to daisy chain thunderbolt displays, though from what I've heard its doesn't work currently even with the Apple Thunderbolt Display.
There's also usb->dvi adapters -- I've got one for hooking 3 displays up to my mini. But the monitor hooked up to that won't get a lot of acceleration -- both 2D and 3D. The dock icon scaling doesn't even look as nice -- I use minimum size dock icons with max magnify, and the # messages badge on Mail.app looks crufty when using the adapter.
NOTE: Using *any* external monitor on the 2011 iMac puts the GPU into a higher power mode (10w -> 30w). This happens when using VGA, DVI, HDMI, or even thunderbolt connections.