Hangarback Walker enters the battlefield with X +1/+1 counters on it.
When Hangarback Walker dies, create a 1/1 colorless Thopter artifact creature token with flying for each +1/+1 counter on Hangarback Walker.
, : Put a +1/+1 counter on Hangarback Walker.
6/22/2015 The value of each X in Hangarback Walker’s mana cost must be equal. For example, if X is 2, you’ll pay to cast Hangarback Walker and it will enter the battlefield with two +1/+1 counters on it.
6/22/2015 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.