If enough -1/-1 counters are put on Toothy at the same time to make its toughness 0 or less, the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many cards you draw. For example, if there are two +1/+1 counters on Toothy and it gets three -1/-1 counters, you'll draw two cards.