system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do elicopters have 3, 5 or more blades, why isn't one number of blades "the best" ? When would 5 blades be better than 3 or 4 or... 10 blades?
gpt
There is not a single definition of "the best".

Fewer rotors means they have to spin faster, producing a different noise profile.

More rotors cost more money.

What's "the best", cheaper and louder or more expensive and quieter?  Like all engineering problems there are many solutions when there are different priorities.