I mean I use at least two; one for vim and a second one mapped to my system clipboard. But I'll often yank to other buffers when I'm deep in a project, sometimes you don't want to lose something you're gonna be pasting throughout a document.

People who make use of macros would be using more than one. I make use of some of the various registers from time to time, but I'd have to be doing something pretty extreme to run out of letters.