The number of flight attendants required is based on the number of seats, not passengers.
Hi Wino...
That is true of many countries in the world, but there are a few exceptions.
Yes, the US FARs and European JARs require that there be one flight attendant for every 50 passenger seats (1:50) or portion thereof installed in the aircraft.
However, in Canada, the CARs require that there be one flight attendant for every 40 passengers (1:40) or portion thereof on board an aircraft. There are additional requirements that in effect dictate a minimum staffing level based on the aircraft type/configuration, the number of exits and any special considerations arising from the certification emergency evacuation demonstration.
The Australian Civil Aviation Orders (CAOs) require that there be one flight attendant for every 36 passengers (1:36) or portion thereof on board and for aircraft with more than 216 seats or that has twin aisles, the minimum cannot be less than the number of floor level exits.
The main difference with the various regulatory authorities is that the Canadian and Australian requirements are based upon the number of passengers actually on board whereas the U.S. and European requirements are based upon the number of seats installed in the aircraft, whether occupied or not.
CAR 705.104 - Flight Attendant Requirements
CAO 20.16.3(6) Cabin Attendants
Kaptain M...
Here is one interpretation as established by the lawyers some years ago here in Canada:
Advisory Circular 0116 - Infant Passenger Count