Only need to shut down (or just damage) one to be a problem. 2.5 lbs of carbon fiber, wires and motors at 100+ MPH would likely be more than enough to destroy a turbofan.
Well, airports sometimes hire out hawks to keep birds out of the sky, and they intentionally limit the speed of aircraft at altitudes where birds fly. If a multirotor can fly higher than birds (they do), they could pose a significantly greater threat than a flock of birds.
