At the beginning of your end step, if a creature died this turn, put a +1/+1 counter on Sabertooth Mauler and untap it.
If a creature didn't die before your end step begins, Sabertooth Mauler's ability doesn't trigger at all. The creature may have died before Sabertooth Mauler entered the battlefield, however.
The triggered ability triggers just once, no matter how many creatures died this turn.