Mine use about 370W stock or 410W overclocked from the wall measured with a Kill-a-watt.
So you should be able to get 3 on a 15A circuit with no other loads. I wouldn't do 4! (but 4 would fit fine on a 20A!)
Source: I wired in 5-20A circuits and have the 2.2TH of Fury & ANTs running....
Thanks much! So if I'm looking for 10 on the next buy, I'll need at least 2 more 20A installed. That's going to be fun to pitch to my landlord.