wrote
(b) all seats in the vicinity are occupied, which means that there
won't be any otherwise unused mask that one could grab. It seems to me
that exactly one mask per seat will drop from the cabin ceiling, so I
am assuming that the question is valid.

In the Boeing aircraft that I flew,(B-707, B-727) there were four
masks for each three seat row.

Bob Moore
ATP B-707 B-727
PanAm (retired)