I have a small collection of "sculpted" behinds, but I think I'll decline posting any more.
According to Wikipedia (where else?), there are 28 monumental size copies of The Thinker, only about 10 of which were cast in Rodin's lifetime. Rodin willed the right to cast it to the government of France, so I imagine the number will grow somewhat. As for which is the "most original", the University of Louisville and the Rodin Museum in Paris are probably the earliest. The U of L cast was shown at the 1904 Exposition. Btw, the Rodin Museum is well worth a visit.
No telling how many smaller versions exist - probably many thousands.
I know the Maryhill Museum here has the only plaster cast of the reduced version. They have a lot of Rodin's work here. All the body parts from broken pieces, tests and experimenting are kinda creepy.