Rule 103.4e

The Commander casual variant uses an alternate mulligan rule. Each time a player takes a mulligan, rather than shuffling his or her entire hand of cards into his or her library, that player exiles any number of cards from his or her hand face down. Then the player draws a number of cards equal to one less than the number of cards he or she exiled this way. Once a player keeps an opening hand, that player shuffles all cards he or she exiled this way into his or her library. After all players have kept an opening hand, each player in turn order whose hand contains fewer cards than that player’s starting hand size may look at the top card of his or her library. If a player does, that player may put that card on the bottom of his or her library.

Find this number in the current Rules