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.
This is a Mahayana question and I'll answer it as such.

The cosmology of the Amitabha Buddha is described as something akin to an infinite number of parallel universes (or galaxies, the cosmology isn't or wasn't trying to be accurate in the astrophysics), each with a Buddha. The Buddha of each universe arise and disappear to be replaced by a new one, the next one in our universes is Maitreya.

The respect in which Sakyamuni Buddha is Supreme is that he was the most recent Buddha in our world. He was preceded by seven Buddhas.

Via reincarnation you can appear in a universe with a different Buddha, which is the whole point of the Pure Land Sutras. The gist is that progress towards enlightenment in this world is difficult, but easy in the Pure Land of Amitabha Buddha.

In the cosmology there is the idea that there can only be one Buddha in the world at time. Personally I haven't come across a convincing explanation for why. My personal opinion is that this is related to the Precept on making false claims of enlightenment. If you are running a sangha and six monks all claim to have become enlightened and all think they are now the rightful leaders of the sangha, that could be a problem.

The Buddhas of other Lands appear to have intercessory powers in our world, for example the Medicine Buddha (Bhaisajyaguru) vows to provide a number of this-worldly benefits to those who say his name.

On the other hand, there is no limit on the number of Bodhisattvas-- Kwan Yin/Avalokitesvara, Kstigarbha, etc. all can exist in the same universe.

Now if this question is really about finding monotheism in Buddhism, you can find that, too. The Adibuddha and Vairocana have been suggested to be the Buddhas from which all other Buddhas emanate, and syncretic Buddhism from time to time has explicitly or accidentally reincorporated a Hindu style Brahma-- a universal, shared soul that is also the monotheistic god or supreme being.

The evidence for all this is from sutras, and presumably the meditative inspirations and visions of the authors. If that doesn't do anything for you, that's fine, too, it reads fine as a myth-- a fictional story to illustrate the Dharma.