The short answer is, "It depends”. The long answer is that the number of fans you'll need is determined by a few different factors. The square footage of your home, pre-existing ventilation, the pitch of your roof, insulation and sealing of the attic, and whether or not you have vaulted ceilings all come into play.
For example, you'll need one fan per 1,200 square feet if your roof has a low slope. If your roof has a higher pitch, you'll need two attic fans to cover a 1,200 square foot space. For very high slopes, you'll need two units to generate adequate ventilation in an 800 square foot space. (9)