I've heard talk about "alts" and "fragments" and "backups" . An alt is an alternate avatar. a "fragment" (as I understand it) is an avatar that represents just a piece of yourself. I've also heard some people keep another avatar around to save their stuff. I've got three avatars that I use but have created at least six in SL over the years.
So how many avatars do you have and how do you use them?