Bloodtracker

• WWizards of the Coast2018-07-13
  Official Ruling
  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.

Rulings via Scryfall