I flew on a Braniff(remember them) flight about 30 years ago with four passengers, and a crew of four. I liked it.
That happened to me once on a red-eye from LA to Pittsburgh. The flight crew outnumbered the passengers.
I remember, as a kid, flying the West Yellowstone, MT. - Salt Lake City route during the non-summer months on Western Airlines. Near the end of the route (and the airline), there would be usually my dad & I and 1 or 2 other passengers. You knew the end was near.