I wish the idea had stayed that simple because it got way out of hand very quickly and once I got going there was no stopping me. It's the engineer in me. Now that you understand where I'm coming from, let's move on. The number of fans really depends on how much airflow you want, how hot your case is going to run, and how much noise you're willing to put up with. Some common sense rules play in here.
Cool air sits low and hot air rises.
The best way to configure your intake fans is to have them pull air into your case from the front and down low. Then as the air travels into the case it's going to warm up and it's going to start rising. So your exhaust fans should be placed high and towards the back of the case. Keep in mind that you want airflow to travel across the entire case and the best way to do that is diagonally. I'll talk more about those considerations later.