Why do ceiling fans in the US have 4 or 5 blades, and fans in India have 3 blades?

 In US and Canada, the primary requirement for the ceiling fan is to supplement the air conditioner in summer. In winter, many of these fans with a reverse switch, can be flipped to turn the direction of the rotation so that it will draw cold air from below so warm air from the heater (usually located along the walls close to the floor or with vents on the floor) can fill the room underneath the fans. So, the ceiling fan keeps a gentle air circulation in the room.

Also, keep in mind, the height of ceilings and hence volume of rooms are quite low in US/Canada homes ~ 8 ft high compared to 10–12 ft in South Asia.

Typical North American fan:

The fans with 4 / 5 blades are slower. Also, these fans generally have a maximum of 3 speed settings. Also, the fact that they almost always double up as a lighting fixture with 1–4 bulbs under the central hub could be one of the reasons why they need to be slow and stabilized so as not to blow the lights.

The Indian fans have about 5-6 speed settings, are not designed to run in reverse direction and seldom double up as light fixtures. In India, the ceiling fan is used primarily as a stand alone appliance that does not change the actual temperature of the room but produces a perceived cooling effect by forced convection of air in the room.

Typical Indian fan:

To that effect, it just has to blow wind and operates at higher speeds than the ones in North America. So, the 3 blade fans are lighter, operate at a higher speed and are generally efficient.