text: The short answer is that three blades do the job.  Even the largest wind turbines using blades nearly a football field long commonly use three blades.  Although four or five blades may work better, the additional improvement likely does not justify the added weight and cost.  Also consider that there is a "wake" behind each moving blade and depending on rotating speed and air velocity, too many blades would prevent each blade from biting clean air.  As a fan becomes larger, the circumference at the tips is also larger and perhaps larger fans can use more than three blades efficiently.  I just noticed that my window fan uses five blades and is much smaller than a ceiling fan but rotates much faster (higher air velocity) on high speed.