im wondering the same question.. im going to take a guess here and say 2? since there is two thunderbolt ports
After.... the very trusthworthy people in everymac.com, the iMac (late 2012) can drive two TB displays with resolution up to 2560 x 1600
Six displays maximum maybe? I may be wrong though. 2 thunderbolt ports to 2 HDMI adapter cables. 4 usb 3.0 ports to 4 HDMI adapters. You would need 4 of these and 2 of these.