8, 13, and 20 are the only ones that I ended up guessing Brian's butt for, but that's mostly because I had no clue at all!
I thought they were all going to be Justin, but then I was sure 13 was Brian because there's something on his left wrist and at first I thought it was his bracelet, but then I saw the bit of shirt (?) just above his waist and thought maybe it was a watch and it was the doctors appointment when he found out he had syphilis.
I think I could change #12 to 101 though. It's hard to tell the shower scenes apart :)