Some hotends need or benefit from a fan to cool the cool end for example the E3D. Some materials such as PLA use a print cooling fan to minimize warping.
So, depending on your hotend and material tro print you may want 2 fans. One is directed on the cool end of the hotend as in [
e3d-online.com]
The other is directed onto the print possibly with ducting. If you post some details of your planned printer then people could help more.