There can only be one God by definition. If you just define "god" as a "super human spiritual being," then sure there could be multiple "gods." On that view, angels and demons could be called "gods." The OT had no problem calling false gods like Baal and Chemosh "gods," because that's how they defined the term--just a super human spiritual being.
But when you then start talking about "God" as the Bible defines Yahweh, then there can be only one. "God" here is understood to be the sovereign Lord over all and Creator of everything. He is omnipotent, omniscient, perfect, etc. There can only be one such being. The reason is simple enough. Suppose there were two Gods (properly defined). The problem is that they would be absolutely indistinguishable, and that which cannot be distinguished from something else is to be regarded as identical with it. Let me explain that a bit more with some examples.
Regarding the rule on indistinguishability entailing identity, think of yourself. Make a list of your characteristics, your "attributes" if you will. This is what I know about you:
1. FallenFromGrace is a Christian;
2. FallenFromGrace has a Norse friend;
3. FallenFromGrace has an interest in philosophical questions about God;
4. FallenFromGrace is having some doubts about his(?) Christianity.
etc.
Now, I don't know your mother's name or where you are in the birth order, so I'm going to make that up, so forgive me if I'm terribly wrong and make the necessary adjustments. Let's call your mother Grace, and let's say that you are her first born son. Now, let's consider the attributes of this person, "The first born son of Grace." We know about him:
1'. The first born son of Grace is a Christian;
2'. The first born son of Grace has a Norse friend;
3'. The first born son of Grace has an interest in philosophical questions about God;
4'. The first born son of Grace is having some doubts about his(?) Christianity.
There is obviously nothing to distinguish FallenFromGrace from the first born son of Grace. If this list was exhaustive of ALL of your attributes, then we would say that FallenFromGrace IS the first born so of Grace.
The rule is, in order to say X and Y are different, distinct beings, there must be something different and distinct about them. You must be able to say that this is true about X but it is not true about Y, and that is true about Y but not about X.
So, back to your question. The problem, you can't say that about these multiple Gods. Let's take "perfection" as an attribute. To be "perfect" means "to lack nothing." To perfect beings would have absolutely all "perfections" (in philosophical jargon)--neither of them would lack anything at all. But in that case, you could not say something was true of the first God but not of the second, because if you could do that, you would be necessarily saying that there was something the first or the second being did not have (so that you could affirm the distinction); but in that case, one (or both) would not be perfect by definition. Thus, they must be identical.
The same case can be made with regard to God being the First Cause. There can only be one First Cause, because if there were two First Causes, then either
both are the First Cause of everything (in which case there is no way to distinguish them),or one is the First Cause of only some events and not others; but if one is only the First Cause of some events, then one is not the First Cause at all (by definition).
More could be said, especially when you start getting into properties like sovereignty, aseity, and divine simplicity, but I think this should be more than enough to get you started.