No I just gamed this out, the magic number is 11, and that incorporates the fact that either the Padres or Diamondbacks are guaranteed to lose 2 times.
Like if the Cubs go 10-7, the magic number will be 1 because the Padres and DBacks can both still win 93 games even accounting for one of them losing 2 games, but they'd have to go undefeated in all games against other teams, and neither team can sweep the other in that final 3 game series.