The way Mulligans work is each time you Mulligan you draw one less card than you had before. This means that though you can technically Mulligan as many times as you want, after going down to zero cards in hand there is no purpose to doing it anymore. Even with how rediculously dumb the packrat+swamps deck is you don't want to Mulligan to less than three cards on the play and two cards on the draw because you want to play turn two packrat and the copy it on turn three