I usually figure 5 amps per computer (600 watts), I always overestimate because popping a circuit with a computer on it is a bad idea. If I have too many computers to power, then I will actually calculate how much power draw that is being used and distribute accordingly. I don't care if I break the cirucit that monitors or other equipment are on so I group those together if need be. I have tapped into lighting circuits with connectors to get more power. One time I even unplugged my main refrigerator (dedicated circuit) and used my mini fridge for drinks so I could squeeze a couple more computers in. I've since split up my wiring onto more breakers so I don't have that problem again.