Rules p. 14 wrote:
Note: Defeating any Mastermind Tactic card
lets you rescue all the Bystanders currently
under that Mastermind and put them into
your Victory Pile.
Defeating the Mastermind is basically a shorthand for defeating a Mastermind Tactic, so if you have for example Hawkeye's impossible Trickshot which lets you rescue three Bystanders for each Villain and Mastermind you defeat and you have enough attack to defeat the Mastermind twice, you get 6 Bystanders from the pile (and if the Mastermind had Bystanders, then you get those after the first attack).