My fans pull air in from a vent at the front, and blow air out at the back. This may or may not be the best solution I'm not sure, but you might be advisable to ensure air flows throughout the case - check which way the air is going by putting your hand near the fan (Or the way the blades are facing if you know that sort of thing)
One at the front and one at the back should suffice, things won't get too hot as long as you have fairly meaty fans on your CPU and GPU, though if you have a lot of drives and cables knocking around you could probably use a little extra air in there.