I HAVE THAT CASE...
It comes with 2 fans:
140mm fan on the top
120mm fan on the back
(Both have adjustable speeds)
but for the best airflow, I have installed another 2 (120 mm) fans in the front and 1 (120 mm) fan on the side.
My system stays cool and I have my cpu overclocked to 3.6GHz.
Would you need to add more fans... no.
The two included will pull cool air into your computer and push hot air out. Any extra fans will just increase airflow even more which is great if you plan on overclocking.