Do you have the case fans now?
One way or another, you will have to try various combinations to get it right....depending on how important temps are to you. The more worried you are, the more fiddling required.
A 4th, 5th, or 6th fan may not help.
Generally, in from the front, out from back. If you have 2 fan mounts on the front side, probably use both of them as intakes. The second one may not help much.....experiment will tell you.
Case fan setup could be complicated if you have liquid cooling. I'd try exhaust first if I had fan mounts at the top of the case, but they may not have much effect.