Song in my head . . .
 

Okay, I have this song in my head.

The lyrics I think go something like "this is (or 'here is') a song about (or 'the story of') number one (maybe 'only one')" The line is repeated at the beginning and end of the first verse. Then additional verses are about bigger numbers, two and three.

It is a slow sort of acoustical thing, the sort of thing James Taylor might have written (I don't know if James Taylor actually wrote it, just that sort of thing).

I am not sure if I have ever heard the original. My memory is hearing a group like a chorus sing it, and also of hearing a male singer like James Taylor sing it. Again, these may be misremembering or covers rather than the original.

It's heck to search for this on google, because I keep getting children's songs about numbers. Also the lyrics I remember seem to be fairly common to lots of songs, and I don't seem to have the precise lyrics)