If Aetherborn Marauder steals counters from multiple creatures, does Winding Constrictor see a single ability putting counters on the Marauder? Or does 121.5 mean that each ‘move’ happens separately, causing Winding Constrictor's replacement effect to add additional counters equal to the number of creatures stolen from?

Aetherborn Marauder's ability removes counters from multiple separate objects simultaneously, and places them all on the Marauder simultaneously. As such, Winding Constrictor will see only a single instance of counter-placing being performed, and will only add one to the total.