Hinduism is usually the poster-boy for polytheistic religions, but from a class I took in college we learned that actually all those gods with multiple arms and elephant heads really sprung from a single one god, called Brahman.
Now Christianity is usually considered to be Monotheistic. But then you learn it has a triune god, plus lots of angels and in the Catholic flavor, saints with super powers! (I heard St. Nicholas, aka. Santa Claus, was actually more powerful than Jesus in terms of raw miraculous razzle dazzle.) Well, that sure sounds polytheistic to me!
Then there is the standard assumption when you study world history that the progression from religions with many gods to one god was some kind of progress up the "evolutionary" ladder of civilizations. Obviously we should continue that progression from one to zero!