Everyone can count to one. It's the easiest thing in the world. There's nothing to it. One. There. Done. It's easy to do and it's easy to recognize. But is it? When we see one thing, is it really one thing, or is it many, or is it none? It really depends on what you want to "use" it for.
Example. One train. But it's made of several parts: cars, seats, materials, molecules, atoms, and more as you go. Maybe even people, history, words, memories, depending on your view of what "things" are. It's not really one thing. It's many things in one spot at one time (and another spot at another time). Why is the "one train" the one thing? And not all of it except for one car? Or just the inside and not the outside? Or just the red things? Because we don't "use" those things - we use the whole train. We use that collection of things for one purpose: to get to work, or school, or to the mall. We look at the schedule and see that that collection of things leaves at 7:23 a, so we had better be on it (or some part of it) around that time or we'll be late. We use it as a whole so we name it as a whole so we see it as a whole.
It's definitely multiple things, but is it also nothing? Well, not with the same meaning of "thing", of course. But we use it as one thing, and we can't really use it without all the other parts of our experience (the tracks, our self, matter, other people, our asses, time, etc.). The train is really a pretty seamless part of the one thing that is our environment, the universe, the stage we play our lives on. In a way, it's an indistinguishable part of the world and it's not really a substantial thing on it's own. So it doesn't really count as a whole "single" thing.
So nothing's really single. So cheer up all you lonely people!