I am relatively new to this sort of thing, but I'm going to go ahead and say if you can pull cold air from outside (intake) and push it somewhere else (out-take) and also have a 600 cfm fan pushing the air out, you most likely could get away with one 600 cfm fan. They are pretty expensive, but you will need it. Right now I am running one 600 cfm with a 1000 watt lamp, soon to two 1000 watt lamps, and I was told I could get away with the one fan for a little while if I am pulling cold air at all times. Again, I am sort of new at this but I have experienced no problems thus far...

In short, go to your local hydro store and buy a 600 cfm fan that is quiet, and see what that does for you. If you need to add another, add another (all grow rooms and locations are diff.) but I would try the one for now... best of luck