Geek alert: only theology & philosophy nerds should read this post. (It is a distillation of one swath of my study project for comprehensive exams.)
In Question 11 of the Prima Pars of Thomas’ Summa Theologiae, he treats the question of the unity of God.
In this particular section of his “Treatise on God” (usually considered to be questions 2 – 26 of the Prima Pars), he makes statements which, “by good and necessary inference” allow the reader to conclude that God is not numerically one.
But to see this, one must first take a quick plunge into the way that the ancients thought about number, for upon this way of thinking, Thomas is wholly dependent.
Two quick points to make here: 1. that “one” is convertible with being; 2. that “the numerical one” is different from “the one that is convertible with being.”
First, that oneness is convertible with being. Thomas, in question 3 of the Summa, adumbrates the simplicity of God: that God’s existence is his essence, and that God has no (non-metaphorical) predicate that is not also his essence. If we can say “God is good,” for example, then it is necessarily true that God is goodness. So also for “one,” “beautiful,” “real,” etc. [By the way, an interesting corollary of this doctrine is that we can be sure that, in a meaningful sense, God is not angry. See this post.]
Because God is simple in this way, it is impossible that he exists “through another,” which is the medieval (and ancient) way of saying that he is uncaused. But if he is uncaused, then must be necessary. Right: God does not exist contingently, like material beings, but rather necessarily. (Note: Averroes believed that a) material beings, i.e., the celestial bodies, exist necessarily; b) that effects, like Plotinus’ Nous and the heavenly bodies, can exist necessarily. Thomas disagrees with him, agreeing with Avicenna that spatial extension is convertible with contingency.)
All of this means that God is what you might call “full being.” Or “Being itself.” Or “Being as Such” (as long as, by that last denomination, you don’t mean “Being in General:” shame on you, Francisco Suarez).
Now, if you like Thomas Aquinas then you also have to like Parmenides (at least in a qualified way). Thomas, like Plato & Aristotle before him, gives Parmenides a qualified “high five” for his insight that being must be one. If two things exist (Aristotle & Thomas would say, “… exist in the full and proper sense”), that is, then this necessarily implies “privation,” or what Parmenides calls “non-being” (if for no other reason than that “A” is not “B.”).
But … what do we (or does Parmenides) mean here by “one?” Thomas think, in Article 1 of Question 11, that he means “undividability.” That is, the one thing that exists cannot be “sliced and diced” such that you can chop A in half and get two A’s, two of the same thing. This is how being must needs be for Parmenides: undividible.
One more point. In this article Thomas also teaches (following Avicenna) that this kind of oneness is different from numerical oneness. The latter, he thinks, would imply an actual numeric infinity (off limits for him), and would “add something” to God in the same way that white “adds something” to the substance of Socrates.
Hence, for Thomas (and for me) God is one, but God is not numerically one.