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If enough -1/-1 counters are put on Bloodtracker 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 three +1/+1 counters on Bloodtracker and it gets six -1/-1 counters, you'll draw three cards. That's because Bloodtracker's triggered ability checks the creature's existence just before it leaves the battlefield, and it still has all those counters on it at that point.