Someone IS a wienerbucket. The bucket represents the vagina. Multiple penises have been inside the vagina. Thus, making the vagina seem like a bucket that "wieners" have been placed in.
This does not include multiple penises being INSIDE the vagina at the same time. Only past and current partners.
Hank: Go ahead. That girl's a total wienerbucket.
Jimmy: Nevermind. Your sister is much more appealing anyways.