My memory from flying the -8 at another company is that the limiting factor is the number of descent reels on board. Which is 8. So the limitation is on upper deck occupants and not necessarily on jumpseats. So, if you have a 2-person crew operating ANC-SDF, there should be 6 available cabin seats.

Definitely makes no sense why Boeing didn’t just put 10 reels on the thing. But I’m no engineer....