Originally Posted by After Shock Media

I thinks the following:
1 person - solo sex.
2 persons - sex.
3 persons - three way.
4 persons (2 m 2 g) - group sex.
4 persons (3 m 1 g) - gangbang.
5+ persons (near equal mix) - orgy.
5+ persons (4+ m 1 f) - gangbang/train.

Or (can swap sex here):
1 female - masturbation.
2 females - girl girl sex.
3 females - three way.
4 females - group sex.
5+ females - orgy.