I had two in a 2x3 foot tent, and was shocked to read that the manufacturer thought more could fit. I topped, and those two girls absolutely filled the tent. I also had 400W. So for 600W and a little more room you might be able to work up to 4, but I'd start with 2 because as they flower, you do have to figure out how to service them without moving them too much -- their stalks end up leaning on each other for support in a big way and a small adjustment can easily be a big mistake.
I'd practice with two and then increase from there. Good luck!