I agree with Liam, a common scenario is using a single flute to do the roughing part, and then switch to a 2-flute ball endmill for finishing the curved sufaced: since finishing removes very little material, and probably not in bottom of deep slots anyway, as long as you have good chip evacuation (a jet of air or dust collection in place) there should be no problem at all using a 2 (or 3) flute ball endmill.
I remember using a 3-flute #202 ball endmill for finishing in some of my early aluminium experiments, not a particularly great example of a good finish, but just for illustration.
I also remember someone recently commenting that they did not really like their rare single-flute ball endmill, but I can’t find that thread right now.