I just rewatched the episode and drew some conclusions about troop numbers from Robb's map pieces. Hear me out.. There are 3 wolf pieces ( Starks), 2 X ( bolton ) and 4 towers ( Frey ). If we consider each piece to be work 2k soldiers, this works out quite neatly. Starks: 6k Bolton: 4k Karstark: 8k Frey: 8k Karstarks are 8k by subtracting Stark and Bolton from the 18k forces after Robb sent 2k off to die. Also it fits very will with the fact that Karstarks are nearly half of Robb's forces. Freys are 8k considering four pieces ( 2k each ) and also the fact that Robb says that he needs troops to replace Karstarks and only Walder Frey has that sort of army which can match the Karstarks. .. I know, this doesn't take into account Robb's other bannermen. However, Robb doesn't have any bannermen in the show. Other than that season1 episode. In S2 and S3, he only has Karstarks and Boltons. Edit: Tullys have no significant troops either. Poor Edmure.