One should be plenty, assuming you are thinking of something like this
You just leave the flaps open that have screens, that acts as a passive air intake. The fan is going to be strong enough that you will see the sides of the tent suck in a bit. Get the speed controller, 50 to 75% should work and will keep the fan quiet enough to not bother anyone. Even the "quiet" models are pretty loud at full power.
If sending air out the top, the bottom vent is key. You are trying to pull warm air out with the fan and then using the vacuum to suck fresh cool air across as much of the space as possible.