You only NEED one fan. The more the merrier though.
The included fans and fan options for the Fractal Design Define R5 are as follows:
Front: 2 – 120/140 mm fans (included is 1 Fractal Design Dynamic GP14 fan, 1000 RPM speed)
Rear: 1 – 120/140 mm fan (included is 1 Fractal Design Dynamic GP14 fan, 1000 RPM speed)
Top: 3 - 120/140 mm fan (not included)
Bottom: 2 – 120/140 mm fan (not included)
Side: 1 – 120/140 mm fan (not included)