The biggest deficit that a team ever overcame to win a game is 12 runs. The Tigers were down 13-1 going into the bottom of the 5th inning on 6/18/1911. They won the game 16-15.
The A's also came from 12 runs down when they were down 14-2 in a game on 6/15/1925. They won the game 17-15.
On August 5th, 2001, The Indians completed the biggest comeback in MLB History. Being down to the Seattle Mariners, 12-0 in the 4th, and 14-2 in the 6th, the Indians rallied to win, 15-14 in 11 innings. Thus, it became know by baseball fans as The Impossible Return.
The biggest comeback pn the ninth happened on 5/23/1901. Cleveland was losing 13-5 in the bottom of the 9th, 2 outs, no one on base. They then scored 9 runs to win the game 14-13.
So the most anyones ever been down and cameback was 12 runs.