One thing that I’ll add to the previous answers …. Even if the FAA says that the plane can be flown single pilot (I have a Cessna Citation type rating and Single Pilot endorsement) … the FAA might say you can fly Single Pilot, but the insurance companies come along and might say something different. As a single pilot operator of a go-fast plane, the insurance companies made the recurrent training and time-in-type bar so high that it was financially easier to put a second pilot in the plane. We were using the plane in a Part 135 operation, so the passengers were basically paying for the flights. It didn’t make sense to fly single pilot. But we also were using our plane to train a rather wealthy individual that wanted to purchase his own Citation and fly single pilot. He trained with us for over 200 hours before he finally purchased the plane and contracted with us for a second pilot when he went flying. His buddies were still impressed seeing him at the controls when they jettted off to Vegas on the weekends. He got the street creds he was looking for. Even thought I was doing most of the flying .. this guy was scary as shit in anything other than VFR flying at altitude. But I wanted the jet time so I flew with him. Looking back, not the smartest thing I’ve signed up to do!