But seriously, I think they usually get one and the others are for the signed copy giveaway on JJM's twitter.
It depends on the publisher, the license, and the contract. Some you get a case (and that tends to be the way on these big licenses); others, you get a handful. On my role-playing stuff, it was just two copies. There are extremely rare occasions where you have to go hunting to get a copy at all; I just completed my own collection of comics I've written by chasing down a small-press book.
I also order books to make sure I'll have enough for shows, publicity, and autographed book requests; both the contributor copies and the ones I ordered have now arrived, and part of today has been spent getting copies to folks who helped. (In the case of books not yet out, like this one, I do take preorders for
signed copies like many authors do; I don't ship until the release date, but all the ones coming in today are getting the 50th anniversary date in the inscription.)
And here comes a delivery truck down the road. No idea what this is...!
[EDIT: Aha, it was the
Halo book I'm in with James Swallow, Christie Golden, and many other fine authors. Busy week!]