Files which need to be shared and stored accordingly get their own space; for instance, in my previous position, I was a Student Services secretary. There were 2,000 student academic files in my possession, but they weren't "my" files, obviously, and lived in a centralized filing system rather than in my drawers.

When it comes to your own files, David suggests you should break things out into a separate drawer when you get to half a file drawer's worth of files on that one topic, which I've found to be a good rule of thumb. Right now, I'm lucky enough that my general reference system all fits in two file drawers. I have one drawer that is A-Z with just general stuff, and one drawer that is only for classes, conferences, and papers for my Ph.D. program.

If there are only a handful of files in any given category, you can just put the category on the label and file them A-Z. For instance, I'm learning a new hobby of photography, and so I have a few files like "Camera—Rebel," "Camera—Powershot," "Camera—Outdoor Photography" etc. It keeps them bunched together in the drawer, but there aren't so many that they can't just all get filed under "c"