I find the Bible to display a bit of a broader perspective--or, perhaps, several different perspectives--than a simple term like monotheism.
In one sense, as others have argued, the Bible makes clear that there really is only one God. Belief in other "gods" is a joke, because those would-be gods are made by the very people who worship them. This is the perspective in, for example, Jeremiah 5:7: "Your children have forsaken me and have sworn by those who are no gods." (ESV) In this sense, the Bible displays a fundamentally monotheistic perspective.
On the other hand, the Bible talks about other "gods" in more than one way. For example, I found 9 cases in which God either warns his people not to worship other gods, or is angry at them for doing so. In these cases, they are spoken of not as idols, but as gods. Likewise, there are over 100 references to "my God", without denying that "my God" is the only god anyway.
Or see Joshua 24:15 (ESV):
And if it is evil in your eyes to serve the Lord, choose this day whom you will serve, whether the gods your fathers served in the region beyond the River, or the gods of the Amorites in whose land you dwell. But as for me and my house, we will serve the Lord.”
Don't hear me to be arguing that the Bible is claiming that these gods are the real deal, or that scripture is advocating henotheism--in none of these cases is the Bible claiming that other "gods" truly are gods. However, the perspective that seems apparent is that inasmuch as they are worshipped as such, they are de facto gods to the people.
So, I would say that the Bible has a broader perspective than you might imagine. Ontologically, scripture is clear that God is the only God, and Christianity (and Judaism) is thus monotheistic. However, the perspective of some passages allows that, functionally speaking, men do worship many gods.