It varies a bit with configuration, but if you look at the MMEL it becomes clear that there need to be 2 ways out of the upper deck. On the 400F that is one slide and one reel per crewmember.

On the 400C (BCF) you still have the two slides, so all the reels can be inop/missing if both slides are servicable. If one slide is inop then you are back to needing one reel per crewmember. (This is why the BCF can carry more bodies than there are reels -- if both slides are operative.)

On passenger birds an inop slide gets into limitations on the number of pax on the upper deck, and I assume that is the number that can get down the stairs and out a main deck door in 90 seconds. In any case that brings you back to needing the reels for the operating crew.