Why do some helicopters have different numbers of blades?

Answer 1

If you mean rotor blades, theoretically, only one (provided it is counterbalanced and moves fast enough). But you'll typically see two or more. Helicopters use more than one because it requires less rotor speed to get the same lift, and it also runs a lower risk of failure in the mechanical systems. Many rotocraft use four or five, and some up to six or seven blades. There are a few helicopters that use two main rotors, and they are counter rotating so that their torque cancels and a tail rotor is not necessary. But the mechanical complexity of these main rotor systems impinges on mechanical reliability.

Answer 2

The number of blades is one aspect of how much weight a helicopter can carry. From the single rotor bladed Boelkow BO-102 that could only hover to the massive Mil Mi-12 with 10 blades and 4 engines that could lift 213,400lbs almost every design type has been attempted.

Answer 3

The size of the helicopter, it's intended use, and how much weight it is ment to carry are all determining factors in how many blades are used.