system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some helicopters have 2 blades whilst others have 4 or 5?
gpt
The greater number of blades in a helicopter enables it to produce more lift, hence heavier helicopters, such as Sikorsky CH-53 has more blades. 

However, there will be trade offs from having more blades. The helicopter blades produce a wake of turbulence behind them, reducing the efficiency of the subsequent ones, resulting in a need to space them out sufficiently to prevent the turbulent wake from affecting the performance of the other blades to much. Furthermore, a greater number of blades will also complicate the hub, and overall increasing the cost. Hence it is not effective to increase the number of blades in small helicopters.

In order to further increase lift, the blades can be rotated at  higher velocity. Then why can't heavier helicopters just rotate their blades faster? This is due to the tip of the blades limited to subsonic speeds, as supersonic blade tips will create extremely large amounts of drag and turbulence, greatly reducing efficiency. Furthermore, even before the blades reach supersonic speeds, there would be formation of local pockets of supersonic airflow, which also creates a large amount of drag. 

Hence, due to turbulence created by the blades, and the inefficiency of supersonic tips, there is a need for more blades in heavier helicopters. Small helicopters are better off with lesser blades to reduce the need for a complicated hub, as well as to save cost.