Frank Miller has stated that he's working on the script for Sin City 2 and is hoping that there will be a total of five Sin City films.
"I'm finishing the script right now for Sin City 2 - I can't wait to get back behind the camera. If I have my way, there's going to be five [films] - but that's if I have my way! Five would take care of all the graphic novels - but there's new material that I'm writing for the second one. There'll probably also be a separate graphic novel that I'll do. It's a Nancy Callahan story that I've been wanting to do."
My only complaint with the first Sin City was that all the stories felt too rushed, so if they spread the rest out among five films I'm sure they'll be great. Plus if they're sticking with Jessica Alba, I don't see how a Nancy Callahan story could possibly go wrong. Two hours of Jessica Alba playing a good-hearted stripper? We might as well hand over the Academy Award for Best Actress now.
Frank Miller Talks Sin City 2 [Empire]