Thoughts
Mathematics is an arbitrary human invention. The concept of a number "1" is nothing more than a concept.
It is useful, as it relates to the real world, certainly, but single objects don't actually exist in the real world.
If you look at an apple, it's natural and easy for you to draw a circle around it in your mind and label it as a single apple. And once you've done that, it's natural and easy for you to say that one apple, and another apple, is two apples. But that's all happening in your mind. The atoms don't care whether they're in an apple or not. There's no formal definition of when two apples are "together" and become "two apples" vs. "one apple" and "another apple." The concept of 2 only applies to apples because you've chosen to use it.
To quote Hofstadter, "we don't apply number theory to cloud systems, because the very concept of whole numbers hardly fits. There can be one cloud and another cloud, and they will come together and instead of there being two clouds, there will still only be one" (G.E.B. 457).
So we have this concept of "one cloud" in our brain, that makes sense and is useful. But the mapping between the "pure" ideas in our brain and the real world is imperfect.