Are Amitabha and Sakyamuni the same?

Buddha is the name for the Supreme Being, right? So are Amitabha Buddha and Sakyamuni Buddha both Supreme Beings but of different galaxies or something? I assume there are more buddhas than just these two. If so, how can there be many Supreme Beings? I guess I am confused by what it means to say Sakyamuni is the buddha of our saha world while Amitabha presides over Sukhavati.