You have to visualize a Ven diagram with the overlapping circles labelled metal removal rate, power requirement, surface finish, material strength, chip clearance, metal cutting characteristics and probably others.
As Ken says if you are cutting aluminum, which is low strength and can make big chips when cut at fast feed, you may need two flutes for chip clearance.
On steel which is stronger and which has to be cut at a slower feed you can go to four, five, six flutes to increase metal removal rate.
If you want an ultrasmooth finish on a profile on any material you go as many flutes as possible because the cut depth is going to be very small so chip clearance is not important.
I think if you are using a smallish mill which is limited to less than 5000rpm and does not have a big motor it is not necessary to go beyond using a two or three flute standard end mill for aluminum and standard four flute mills for steel.
An open mind is a virtue...so long as all the common sense has not leaked out.