I had a long haul flight from DFW-SYD that had plenty of empty seats to the point they offered an upgrade to guarantee you'd be the only person in the row. Best spent $100 ever related to airfare.

One time I was on a 747 YYZ→LHR with a total of 14 passengers.

I had that SF to Heathrow once, though I recollect four seats? Only time I’ve ever had a lie-flat bed on an aircraft.