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.

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.

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.