Hopefully it's sold a lot, however we got to remember it still is at its heart a more niche title that a cross section of the gaming community would just stone wall out of hand so it would be wise to manage expectations a little
http://store.steampowered.com/stats/ . I'd be surprised if it sold as well as, say, Civ 5 did in the opening weeks, since that has the brand awareness that crosses out of the strategy market a bit more. But I guess that would be the price for developing more hardcore strategy games and god love Paradox that they do.
I suspect whatever the case, it'll ultimately sell, or have sold, more than any other Paradox grand strategy game as it has more mainstream appeal than any previous title IMO. Hopefully enough to justify expansions and such. And as much as I love EUIII and whatnot CK seems like a franchise that could become a break out title that draws in a lot of new fans. I know of at least a few Paradox virgins who have jumped into CKII, so that's promising.
EDIT: Yeah according to the quote it seems I was right. Little surprise as CKII seems like the most 'AAA' hardcore strat game ever.
I'm hoping CK can take on Civ head to head someday in a BF3/COD situation as it deserves the same, if not more, prestige associated with it in gaming.