1) Multiplicity is a property of the physical universe and physical things.
2) God is not part of the physical universe.
3) Therefore, God doesn't have the property of multiplicity. He is singular, which fits with the idea of infinity.
First, I'm not sure whether you are using the philosophical concept of multiplicity or the mathematical one. In either case, there is no reason to assume, and no way to know, that multiplicity does not also exist outside of our universe (if such a thing is possible).
Furthermore, when we attach labels to God such as "good", "perfect", "He", or even the label "God" we place labels on the concept. Thus, making "God" finite, and undermining the concept altogether.
On top of that, by defining God as singular, that suggests that God is unchanging. The Bible tells us otherwise, and defining God as a creator undermines the idea that God does not change.
And again: God's infinity is like imagining an infinite (note the analogy here) blue sky, cluttered up by finite clouds. The clouds need a sky, but the sky doesn't need (or is affected by) clouds. Likewise with infinity being a necessary precondition for finitude, or God being a necessary precondition for the universe.
But the problem here is that you are defining God as "outside the universe" we are stating that reality is made up of at least "The Universe" and "God". God, therefore, cannot be infinite, as He is defined as not-The-Universe. There is something bigger than God: God + the Universe.
In the same way, you say there is an infinite blue sky, and clouds. Since you define clouds as being something different from the sky, the sky cannot be infinite as the sky does not include that which you define as clouds.
If you wanted to say that there is an infinite blue sky and leave it at that, then that would be singular.
And beyond all of that, let's just point out that this is all meaningless speculation. You're just making stuff up.