Noting the charge levied in another comment on this, I’d amend to 5 people with a minimum of two bottoms.

A group of 5 people with 4 tops and 1 bottom probably changes the classification to a gangbang. 1 top and 4 bottoms, a reverse gangbang (and likely tricky to pull off). 2 tops and 3 bottoms, or 3 tops and 2 bottoms, would seem to form the minimum unit to be classified as an orgy. However, further study is needed.

I want to know more about the unhinged performance art, but looking at your username I’m a bit apprehensive.