If enough -1/-1 counters are put on Hangarback Walker 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 Thopter tokens you get. For example, if there are three +1/+1 counters on Hangarback Walker and it gets four -1/-1 counters, you'll get three Thopter tokens. That's because Hangarback Walker'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.