@IvankaNews_ B. There are many clues from gravity. To wit: The way the hair on group A falls and the creases in the shirts of Group B are the most obvious.
@Matt_Pinner Although there are an infinite number of curves that fit the pattern, the simplest one is n(n+1). So 12 is the answer if you like Occam's Razor.
@natism 6, if the back is mostly missing towards from the opening for the body, at least 7 if the shirt is more normal, as one big hole could enable the see through effect.