The last 2 cruises on Oasis and Harmony there's one performance usually on the last night where the Adult Comedy is held in the theatre.
As some have said, Royal maintains 25% or reserves for on bord bookings. Also, on the larger ships there are usually 3 performances each night. On recent Oasis sail in October, me and my son went stand-by and did not think that we could get in as the line was longer than the reserve. No problem, all of the stand-bys got in. There also is a large bar that one can sit around and watch the performances.