As pure act, God can have no admixture of act and potency within Himself. As such He is absolutely simple. As simple, God has no distinctions within Him of that which is per se or per accidens. There is therefore no distinction between his essence and existence.
From this, we see that God cannot be a species contained within a genus. A species is a composite of specific difference added to some genus, which, more generally, is a relation of act to potency. All genera are in potency to some species, which is act, for genera potentially contain specific differences.
Furthermore, God cannot be a species predicated of many individuals. Individuals of a species are differentiated by their existence but have a common essence. Men are all human beings but are differentiated by their individual existence and accidental qualities. But God, who is Existence Itself and thus has no distinction within Him of essence or existence, He just is His own existence, cannot be differentiated in any way.
Therefore, there is and can only be One God.
“I contend that we are both atheists. I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours."
Once we come to a proper philosophical understanding of God, we cannot make such silly statements like the one above. Polytheism and monotheism are not similar beliefs differentiated by an arbitrary number. There cannot be more than one God because God is not the kind of thing that admits of multiplicity. Of course, none of this matters. This phrase will be repeated until a newer and more catchy one becomes popular. And this new phrase will be just as philosophically vacuous as the last.