"I thought the Jews believed in only one God, but even a cursory look at the Old Testament shows they believed in a bunch of them. I was taught the Israelites were monotheistic but is this not so?"
The term monotheism means to worship one God and one alone. The Israelite law and the practices of the state cult were clearly monotheistic. To believe only one god exists is to be monogott. And though the Israelites were officially monotheistic, they were definitely not monogott. They believed there was a plurality of gods in the world. Indeed, Jeremiah writes, " . . . you have as many gods as you have towns, O Judah" (Jer. 2.28).
The two best known gods, other than YHWH were El and Ba'al. Both of these gods were worshiped by the inhabitants of Canaan (modern Palestine). Ba'al was a Canaanite fertility god. Although Ba'al is perhaps the best known god to biblical readers, El was considered by the Canaanites to be their most high god, the chief god in the pantheon of all gods. El was pictured as a beneficent, grey-bearded old man who was called the eternal one and the creator of all things. And Israel was admonished repeatedly throughout the scripture against worshiping these, or any other, "false" gods.
Interestingly, some scholars identify El as being the original object of Yahwistic worship. In Exodus 6.2-3 we read literally, "God also spoke to Moses and said to him: I am YHWH. I appeared to Abraham, Isaac, and Jacob as El Almighty'" (Exodus 6.2-3). In several of the psalms, but especially in Psalm 29, the separation between El and YHWH seems blurred. In Psalm 29 the writer has taken a Ugaritic (Canaanite) hymn and replaced the name "El" with "YHWH" in all but one instance (v. 3). And in Psalm 89.6-7 El is supplanted by YHWH as the chief god of the pantheon. In these passages, and in others, there is some support for the notion the Canaanite El is somehow equivalent to the Israelite YHWH.
However, whether or not the Israelites originally worshiped El as their primary god does not rule out the monotheistic claims of Israel. God has been known by a variety of names throughout history even Israelite history. God is called El, Elohim, and YHWH in the Old Testament, but this plurality of names does not necessarily indicate a plurality of gods worshiped.
During the exodus the Israelites made a golden calf and worshiped it. Many have used this incident to negate the claims of monotheism against Israel. However, a close reading of the passage indicates otherwise. In Exodus 32.5 Aaron builds an altar before the golden calf and proclaims, "Tomorrow shall be a festival to YHWH" indicating that the calf is an image of YHWH. Indeed, there is archeological evidence that YHWH was regularly portrayed as a calf or a bull. Again, it seems the Israelites were worshiping only one god, and that god was YHWH.
Although Israel seems only to have officially worshiped one god (unofficially many individuals choose to worship other gods), they believed many gods existed. However, they claimed YHWH was the only god Israel should worship: "Do not forget YHWH your God and follow other gods to serve and worship them." (Deuteronomy 8.19). This singleness of worship was unique to the Israelites in the Middle East for many years. Scholars generally agree Israel was one of the first peoples, if not the first, to become monotheistic in practice. So, although Israel was not monogott, their worship was directed to only one god.