While there are a number of arguments for the existence of a divine being, none of them require that there be only one divine being.  Why should we think there is only one God, then?

The simplest reason to think there is only one God is the principle of parsimony: Do not multiply entities beyond what is needed to adequately explain the effect in question.  Since only one God is needed to explain the origin of the universe, there is no reason to believe there is more than one God.  The burden of proof would be on anyone wanting to postulate the existence of more than one God to explain why we should think there is more than one God.

While the principle of parsimony is instructive, it is not conclusive.  It is based on probability, not logical necessity.  It’s one thing to say no more than one God is necessary to explain reality, but that doesn’t necessarily mean there is only one God.  After all, only one human is needed to explain how a house got built, but the fact of the matter is that more than one human was involved.  So are there any logical arguments that would logically require the existence of only one God?