Yes, I know that in some childish minds calling Christians Polytheists is somehow a 'win' for your own religion but let's be realistic shall we?
Catholics, although they pray to their saints and to mary, they do not deify them. So no, they are not Polytheists.
Trinitarians, although they deify Jesus, also don't consider Him a different entity to God Himself so no, they are also not Polytheists.
Why are Christians not Polytheists? Simply because they do not believe there exists more than one God.
So please, give it a rest.