I think the word you are looking for is "polytheism". Here is a definition from Wikipedia:
Polytheism is the belief of multiple deities also usually assembled into a pantheon of gods and goddesses, along with their own mythologies and rituals.
Someone who worships many gods is a "Polytheist" or, as the Bible puts it, an idolater!
I hope you have found this helpful.
God bless,
Mike