A 7B amp has the potential to draw 1980 watts when pushing 900w into a 4 ohm load. I'm not an engineer, but it sounds like you'd want to put each amp on its own circuit (1980 VA / 120V = 16.5 A). A friend of mine has a pair of 7B ST monoblocks and has each amp on its own 20A circuit. I have a 4B and a 9B and each amp is going on its own 15A circuit.
Definitely, I think you'll have problems if you try to put all the amps on a single 20A circuit.