It's six. Localization cannot fix that unless they get permission from Marvelous to change the limit-it's a programming thing, not a translation thing. In Japanese, 6 letters can get you up to a 12 letter name in Roman letters. Obviously that does not translate well to English letters to start off, but that's unfortunately not something XSeed can really do anything about.
Oh, that's really interesting, I never knew that. Makes sense now why HM games have shorter name slots...
Six is decent though, a lot of names I pick end up being five letters anyway.