Fascinating thought! What you're saying has to be an element of the foundation. Parmenides and Heidegger thought of the One as Being itself, which is my point of departure.

From there, I imagine a person noticing he or she could pick out elements of the One, what we now call individual objects. At that point, I imagine each object being unique.

Then the person notices two or more objects look alike, like fruit in a tree. The idea of "more than one" follows from there.

(According to Plato, there is one thing that is instantiated multiple times. This leads to the question "how many?")

Thank you very much for commenting and for your thoughts!