Consider this example. A single engine is apparently acceptable for the F-35B, a couple of which just safely flew across the Atlantic Ocean. Yet the new CH-53K helo is designed to use three engines to make the same flight.
When the established reliability rate of turboshaft engines meets or exceeds that of the rest of the propulsion system which has no operational fault tolerance, such as much of the rotor system and gearbox, what is the justification for not using a single engine?