IDW's Judge Dredd is just all by itself. "Volume number" has no application in terms of continuity.
There have been various times "Judge Dredd" has been published by different companies, but the continuity of one company's version wouldn't necessarily mean anything to the continuity of a different company's version. Just like IDW had recently been publishing a Doctor Who comic book, but the version(s) currently published by Titan Comics don't really have anything to do with what happened in IDW's comic book stories.
It would be like trying to put in order the Batman comic book stories published by DC, but jumbling in:
* the main comic book version
* the comic strip episodes
* the comic book stories based on the various animated series
* Batman '66, which is based on the old TV show version
Same characters in that Bruce Wayne = Batman, but they don't all fit together.
If you
still don't accept that, try here:
http://www.comics.org/series/name/Ju...d/sort/chrono/