There is no count given in the books, nor in the show, but I would think that the garrison numbered in the few hundreds, perhaps 200–300. read more
Since the Blackfish died on Game of Thrones, leaving Riverrun with Jaime, Sansa will most likely not be able to lean on their army as the Battle for Winterfell commences. So basically, once more, Sansa and Jon are kind of screwed. read more
Though the Blackfish commands the forces at Riverrun, formal authority is vested in Edmure as the rightful head of House Tully and the Lord of Riverrun, and his men are obliged to obey Edmure, not the Blackfish. As his men allow Edmure into Riverrun, a disheartened Blackfish leaves the battlements. read more