What properties vary with the number of blades on a BLDC fan?
I am not sure whether this applies equally to fans, but for wind turbines (the things that doe the opposite, turn moving air into a rotation) a higher number of blades causes the optimum working point to be at a lower RPM. Hence the USA-type (water pumping?) windmills with lots of blades are slow-runners, contrast them to modern electricity-generating windturbines that have 2 or 3 blades.

Applying this to fans means that a higher number of blades means that it can rotate more slowly.