Monotheism is the belief in a single God. The best known allegedly monotheistic religions are the Abrahamic religions: Christianity, Islam, and Judaism.
Monotheism and Christianity
It is a matter of some debate how "monotheistic" Christianity really is. For a start some versions appear to have three gods - the father, the son and the holy spirit. Bizarrely, for some weird theological reason this trinity is counted as one.
Then there is the question of some Christian communities veneration of saints and especially the position of the Virgin Mary who is recipient of vast numbers of prayers and has many churches devoted to her; and as a result she is obviously a goddess in all but name.
After that we have uncounted numbers of angels. They have their own hierarchies and are sometimes believed to be sent to interfere in human affairs.
Finally, on the other side there is Satan who has his own band of servants and also interferes in human affairs and is prayed to by some people.
So, while the Christian Pantheon is undoubtedly less populated than, say, the Greek one, it is hardly monotheistic. It must also be remembered that, in general, modern polytheists regard their multiple gods as really being different embodiments of one supreme god - a concept very similar to the trinity in Catholic Christianity.