So Cho-Jung Man said that there are 8 S-Class WHs total, including two that are dead (one Edea Florence, the other the one killed by South or one of the moles(?)) and one that is inactive (Vihyungrang). He also mentioned that one person is acting as a S-Class WH at the moment.
From CJM's Q&A post: http://blog.naver.com/philowliz/60145128941
He also notes there that Tasha still has a year left before Aria's curse activates. Alcatraz will be important to the story later on. CJM hopes to release a world map in the appendix of one of the volumes (if he hasn't already by now). Eunryu's and Lancelot's "stopped aging" status will also become important/will be revealed. Tania is confirmed to be a powerful witch. Dexter's full name is Fedora Dexter.