How many case fans should I have in my Computer Case?

Well if you had included the case name we could give a better answer! Most cases need at least 2 fans one in the front blowing into the case and one in the rear blowing out of the case. The size would be determined by what ever the hole spacing is in your particular case. I personally like "Variable Speed" fan as I can change the RPM to move more or less air depending what I'm doing with m computers. Small (80mm) high speed fans a top speed move a lot of air BUT they sound like a jet plane taking off! The bigger 120mm fans can move the same amount of air at slower speeds so as to NOT make all that noise! With the Variable speed fans I can set the speed (and noise) low for general work and hi for gaming! Some cases have places for 6 hi-speed fans and that's great IF you don't mind having a hurricane in your room that sounds like your just behind a big jet as its taking off! Fan measurements are given in three basic ways. 1. RPMs 2. CFM..the unit of measurement of air pressure (moving air) the higher the better! 3. DBa..the noise level! the lower the better! Be more concerned with CFM and DBa you want high CFM with low DBa. Most cases come with fan in them but the quality of the fans may be dubious to say the least! I have the Cooler Master mini tower case (541) which does not come with any fans. It has the ability to accept a 80MM or 92MM or 120MM in the front and a 120MM in the rear. I mounted the Thermaltake 120MM adjustable in the front and rear. Works good for me!