Hangarback Walker
Artifact Creature — Construct
This creature enters with X +1/+1 counters on it.
When this creature dies, create a 1/1 colorless Thopter artifact creature token with flying for each +1/+1 counter on this creature.
, 
: Put a +1/+1 counter on this creature.
Illustrated by
Edge of Eternities Commander#139RareEnglishLegality
Rulings
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.
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.