The difference between monotheistic and polytheistic religion is that one worships a single god whereas the other worships more than one god.
In a monotheistic religion, a single god is worshiped. It is easy to remember this because mono means single or one therefore showing that it is one god.
A polytheistic religion worships a collection of gods. This is known as a pantheon.
Hope I helped and have a good day or night :)