We're running 2 machines per outlet. I used a KilloWatt to measure amps, 5.35. So that's 2 per 20amp circuit to stay below 16 amps 
We ran 7 additional circuits into the room :p
We ran 7 additional circuits into the room :pBe sure to switch the breakers on and off with something plugged into each outlet to see which ones are on the same breaker...sometimes they switch around which "leg" is feeding it.
If you are running more than 4 rigs I recommend a 240v line with its own breaker...cuts the current in half...just be sure to either use special 240v plugs so you never accidentally plug in something that should be 120v. (You can also make 120v plugins off the 240v sub-panel if you run the 3 wire with the neutral..be sure to use your grounds...breakers/fuses save equipment...grounds are there to save lives).