Well, I'm counting a character as being written if they appear in a story. Jayne is in three fics, so is Book, but Jayne's appearances are throwaway lines in someone else's story while Book's are in fics which are about Book. (The Firefly crew all live on a spaceship together, so it's hard to get away with not writing any of them.)

Of course, you write your pet characters in a setting almost completely divorced from the majority of the canon cast, but I think that's actually quite rare.

PArt of it is community; I hang out in areas (femslash_minis in particular) where exploring the various interstices of the text, leaving no stone unturned, is in vogue, so I've gotten into the habit of writing minor characters a lot. I started out with a more narrow palette, although still more narrow as some (since I started in the fandom writing sprawling epics: Drusilla gets her soul while Buffy and Dawn hunt her while Sam and Riley explore Africa while Faith goes to L.A. while the Watcher's Council pulls together while a thousand things happen in flashbacks).

And of course even with a large dramatis personae, there are characters I keep on coming back to again and again.

It's also partially a question of output. I can say I've written 37 Dawn fics because I've written a whole lot more fics than that (I'm including ficlets and drabbles here, so it's not a hugely impressive number, but still). Someone who writes a fic maybe every six months just isn't going to be able to write as many characters unless a lot are packed all into one fic (again, a sprawling epic).