I would say two Witchers should be the absolute max. 

I wouldn't mind rolling up an ol' cat eyed killer, but can just as easily go for something else if there's a lot of competition for the roles.

EDIT: What's the word on player controlled monster characters? I know some species don't lend themselves easily to the role (fiends, necrophages, gryphons etc jump to mind), but what about the likes of Dopplers or Higher Vampires. They have the sentience required to be considered a viable character, and there is precedent of their like rubbing shoulders with a Witcher.