Hm, as long as you can control them you're free to have at maximum 4 characters I'd say, might increase
the limit

Skeleton key? You mean the character sheet thing in order to sign up, right? If so, probably will have all the basics, plus a few added extras such as greatest fear and so on.

Might have a limit of 6 or so

Still planning a bit on the background story, I'll post it it soon ^^