According to a recent post on the Lionhead community website, Fable 2 for the Xbox 360 has already sold 2,600,000 copies, making it one of the system's highest selling RPGs. To go along with the game's big sales, over 66,000,000 achievements have been unlocked within it. The most unlocked achievement is "The Whippersnapper," with 2,800,000 unlocks, while the hardest to get seems to be "The Dollcatcher," with just 61,037 unlocks.
Additionally, it does not appear that Knothole Island will be the last DLC pack released for Fable 2. The post answers the question of whether there will be any more DLC for Fable 2 with a simple "yes," noting that a development team is working on new content right now. Look for details on this DLC soon.