Without getting into specific real world religions, note that the strict dichotomy between mono and polytheistic religions isn't exactly how this works. D&D is sadly lacking in making use of most of these ideas; but I suppose they've had their share of problems with religious groups already. Still, I think some of these ideas can find a home in a homebrew campaign setting.

You've got a whole lot of different options here.

First, there are nontheistic faiths. Again, no examples -- but there are religions which aren't about worshipping a specific deity or supernatural being at all. Other religions believe there are spirits in plants, animals, and some inanimate objects, and that you should interact with the specific relevant spirit in any given scenario.

From there, we still have a lot of options between "polytheistic" and "monotheistic". Some of these may not sound very familiar; if you're interested in looking into it further offboard, you should. The reason is that while a modern faith is fully monotheistic, now extinct branches of the faith may have had a somewhat different view. Again, I won't get into specific religions here.

So, first we have the DND style polytheism, with multiple groups worshipping different pantheons of gods. Of course, that's not exactly how things always worked in the real world. Often different polytheistic faiths would share gods; perhaps two ancient empires meet and begin trading. Soon philosophers in both nations are coming up with explanations of how their chief gods are actually the same being, just with different names. Gods are traded, borrowed, merged and split.

Following this we have Monolatry and henotheism. In henotheism, you acknowledge that a whole pantheon exists, but you only offer worship to one God. Under monolatry, you only worship one god but believe that other nations may worship their own gods - it isn't a matter of whether they exist or not, but whether your tribe has a relationship of worship with a given diety. We see this come about in a few ways:

1) a god associated with a specific profession might get the vast majority of the worship of people of that profession. A sailor may only worry about the god of the sea. A farmer may only worry about the goddess of the harvest. A king may decree that the only one god from the pantheon should be worshipped.

2) a god associated with a specific city-state or a specific tribe may rise in prominence among members of that group, until members of that city-state greatly prefer the worship of this god over others in the pantheon. In this scenario, they may still consider themselves part of that pantheon, and their people a subset of the greater nation. However...

3) over time, the one deity which a group places chief importance on might rise even higher, to the point where the philosophy shifts. Instead of "this deity out of this pantheon has a special bond with our people and so deserves the most worship" we get "this deity has a special bond with us, and worshiping another deity means forsaking that bond. Other nations have their bonds with their own deities, and this is good. But for one of US to worship another deity would be a terrible betrayal." Think of it this way.... you've got a married couple, John and Jane. If John sleeps with other women, Jane is likely going to be pretty pissed at him. But if she hears that John's friend Jake had slept with his own wife, that's not likely to annoy her. Even if she learned that Jake had cheated on his own wife, she may think less of him, but isn't going to feel like Jake wronged her PERSONALLY. And of course, she expects John not to cheat -- she doesn't expect him to proclaim belief in only one woman. That's closer to the relationship between a monolatric society and their god.

4) from here, we get to a monotheism with an asterisk. Yes, there is only one god, and it is your god. But, there are other beings out there that can influence reality. These other beings might be very powerful- the extreme case being Dualism, where you have two gods who are equal and opposite, sometimes evil and good but other times it is more complex than that. In that case, the evil god is as powerful as the good (and maybe they are worshipped equally). More typically these other non-god beings are weaker. They can be good, or they can be bad, there can be a few or uncountable hosts of them. Sometimes they're responsible for misfortune. Sometimes you pray to them so that they'll intervene on your behalf with the main god. And when you run into a people worshipping other gods, you might conclude that they were fooled by some of these beings into worshipping them instead of the real god.

As you can see, there are tons of options for faiths beyond Mono vs Poly theism, and we just scratched the surface here. I think many of them can work in a d&d setting and hope to see some greater creativity expressed in that department.