Quoting FlagshipAZ (Reply 8):
*It is one F/A per 50 seats, regardless if there is passengers sitting in those seats* |

Quoting FlagshipAZ (Reply 8):
*So for example, a 763 with 216 seats installed still requires 5 F/As even if there is only 108 revenue passengers aboard.* |

Actually, if a F/A is not available for any reason (strike, flu epidemic, whatever), an airline can limit the number of PAX on board according to the number of F/A available.

Example : Your airline is based at

CDG and operates a 168 seats A320.

168 seats = 4 F/A.

Your plane is leaving

CDG to

MAD with 168 PAX and 4 F/A.

In

MAD, during the 1 hour stopover, one of the F/A breaks his leg and is taken to the hospital.

Your plane will be able to return to

CDG with only 3 F/A (the rule of 1 F/A for each pair of main door is respected) but with a maximum of 150 PAX on board.