hey well, the number of fans a case can hold depends on the perticular case. But heres the basic idea of air cooling, you want to keep all of the air moving. so in some cases, 4 fans can do a better job then 20 fans. its all in positioning. the ideal position is intake in the front and exaust in the back to keep steady cool airflow. if you dont have good flow and just a bunch of fans your computer will be really hot.