Chromatic Sphere 
Four or possibly three looks the most correct for this picture, because there appear to be no highlights anywhere else.
There is no one size fits all for this sort of thing. Even from the same angle, different light sources are going to light a subject differently.
My 2 cents, I am not an artist.
There is no one size fits all for this sort of thing. Even from the same angle, different light sources are going to light a subject differently.
My 2 cents, I am not an artist.