What chapter of the Seven Deadly Sins manga is Episode 20? Alright so I am caught up with the anime but cannot wait for the next episode (currently on episode 20). What chapter would match the anime?
Since you said you are currently on episode 20 and cannot wait for the next episode, I'm assuming that you are on season 2, so:

The 20th episode of Season 2 corresponds to the chapters 176, 177, 178, 179 of the manga.

Source:Wiki