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.

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.

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.

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.