*bowl of dark grapes*
Friend 1: I like my grapes how I like my men.
Friend 2: Black? Good one.
Friend 1: 21 at a time.
Listen up, buttercup. The joke is about Friend 1 who likes "grapes" or young "men" 21 at a time. It's funny because it is unexpected and sounds like the friend is talking about sex with lots of people. Hope this helps, dummy.