It only really takes one to whack an engine. The "flock" bit was particularly interesting there because it got both engines at once. A single drone would presumably only take out a single engine. The airplane would likely survive the encounter, since they're built to survive an engine failure. But it would still do a hefty amount of damage (I bet the drone owner doesn't want to be on the hook for the cost of a new engine!) and survival, while likely, is far from guaranteed in the event of a single engine failure.