Generally for fan and propellers:
more blades (or bigger blades) = more force generated = more costs = more weight (which normally means even more costs as the structure will need to be made strong enough to hold the weight)
So the question the engineers designing fans or propellers face is nearly always:
1. Can it do it’s job?
2. If 1. is yes, how do I then make that as cheap as possible
The answer to question 2 above nearly always gives you 3 blades as a good compromise on all factors. If costs is less of an issue (e.g. military) than you tend to get more diverse blade counts. Blade count may also be affected by other factors (more blades tend to be quieter, limited space for the blade, the strength of the material, where you are using the device/structure etc.)
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