Lighter drones don’t need to register whatsoever, and heavier ones (in addition to drones for commercial use) are registered through a different process involving paper (ugh). To put it differently, when you have lots of drones, you should register once possible. A drone sucked in a jet engine will be all around the place. Professional drones aren’t covered with these rules. Even lightweight drones can result in injury should they fall from a substantial height.