It really depends of your GPU, CPU and hard drives.
If you're going to use the Integrated graphics of the CPU and 1 or 2 hard drives those 2 fans included are enough.
Adding a Graphics card, overclocking the CPU and adding more hard drives increases the heat. So if you have a GPU, the 2 x 120 mm fans in the front are a must, (I don't know which Shinobi case you have, if you have the side window version and a GPU, the 120 mm in the side will increase the airflow to the graphics card).
The exhaust is the 120 at the back, and if necessary a 120 or 140 in the top.
Give us more information about your rig to give you a proper way to mount the fans.