Arcbound Wanderer

Modular—Sunburst (This creature enters with a +1/+1 counter on it for each color of mana spent to cast it. When it dies, you may put its +1/+1 counters on target artifact creature.)

Modern Masters

Modular—
Sunburst (This enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. When it dies, you may put its +1/+1 counters on target artifact creature.)

Fifth Dawn

Modular—
Sunburst (This comes into play with a +1/+1 counter on it for each color of mana used to pay its cost. When it's put into a graveyard, you may put its +1/+1 counters on target artifact creature.)

For the leaves-the-battlefield ability, the number of counters on Arcbound Wanderer at the time it left the battlefield is what's relevant; the number of colors of mana used to pay its cost doesn't matter anymore.

This card has modular—sunburst, a combination of two keyword abilities. The sunburst ability sets the number for the modular ability. Basically, the number of +1/+1 counters the Wanderer gets as it enters is equal to the number of colors of mana used to pay its cost.

When the Wanderer leaves the battlefield, you may put the counters onto a single target artifact creature. You can't split them among two or more artifact creatures.

If this creature gets enough -1/-1 counters put on it to cause its toughness to be 0 or less (or the damage marked on it to be lethal), modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.