As far as I am concerned I would confirm what most of you have said, 1 CC per 50 pax seats.
Btw one pax seat is for one adult or children but infants do not count therefore.
So you were legal IMHO.
Otherwise, for example you have 151 or more pax seats but less than 150 pax you still need for CC.
Maybe therefore apply any exceptions. In our company we would be allowed to fly a B737-400 (167 seats) or a B737-800 (184 seats) with only 3 CC when we do not have more than 150 paxes and block the rest of the seats.
Not sure if we are allowed to dispatch this way (the company does not) or this rule is only for the "emergency case" when eg one CC becomes ill ...
There's no prob with the oxygen, for every seat row the oxygen of the number of seats plus 1 can be supplied. The reason therefore is that infants do not have their own seat or a CC might be on the aisle.