If you tour with acoustics then, yes.
Otherwise, just a 6-string and maybe a 12-string if you want to mix it up. Although I got rid of my 12-string because it was a pain in the ass to maintain and keep in tune. (Although those were pre-clip-on tuner days.)
EDIT:
As for me, I currently have a standard 6-string acoustic, and a resonator-style 6-string acoustic, just so I can mix it up a little bit both for recording and songwriting. It's nice to have more than one distinct voice to work with.