The fewer the teeth, the easier the penetration will be
One example would be imbedded rocks.
Were a tooth to line up with a tooth, penetration will be restricted so the less teeth the more likely to fit between rocks and therefore easier to loosen/dig full loads.
Many teeth would work in a 'landscaping' situation whereby you are essentially foreward rakeing roots and that sort of debris.
For basic excavation/loading operations I think you are better off with, say, 8"-10" spacing.
My 48" bucket has 6 teeth and I find it awsome digging in a clay type hardpan.
Topsoil with some rocks is a breeze and nearly always gets full bucket loads.
A FEL with teeth VS one without is just like comparing nite and day!
Only setback is you loose all backgrading capabilities. (that is why they invented back blades)
Naturally foreward motion combined with the proper curling action gives best results.