Why do large windmills and helicopters have three blades, but why do jet engines have so many fans?

Windmills are not fans. Windmills use wind power to generate electricity, grind flour and raise water. The more blades, the heavier the weight and slower power production. efficiency may not be high

Generally speaking, there are two blades. , four blades, three blades and five blades. There are also many blades that can be increased. This can effectively increase lift without increasing speed, so the heavier the blades, the more blades.

Jet engines. require a large amount of air. The compressor blades are used to generate the highest possible wind pressure, so there are many blades