Twice I was the only passenger.
Once RFD (Rockford, IL) to MSP, and once Jamestown, ND to Devil's Lake ND. The ND flight started in MSP and everyone else got off in Jamestown and when I got off in Devil's Lake, no one else got on so it went back to MSP empty.
I was a travel agent at the time flying non-rev also! Both were small 19-seaters.