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From A Kid's Introduction to Physics By Anthony Rizzi, ch. 10:
After explaining "top-down" causality, and how we know there is an "Unchanged Changer" (God) with no physicality or potential, he says:
How many Gods are there? Well, let's say there were two. Now, by saying there are two, we are saying that one is different from the other in some way. This means one has to have something the other does not. This will mean one is not Existence itself but is existence as limited in some way. Thus, this second one is not God. There is and can only be one God.