Buddhism, no matter the branch is neither a polytheistic religion nor a monotheistic one, for it doesn't posit the existence of a supreme God. Tibetan Buddhism believes in the
… existence of deities, but those are supposed to be emerge from Emptiness, and as such are subject to impermanent existence, just like human beings. That alone sets them apart from the gods of polytheistic faiths. Those who follow the Vajrayana branch use deities as meditation aids, a way to develop desirable traits or to overcome undesirable ones. (MORE) 