US Federal Aviation Regulations:
Sec. 121.391 — Flight attendants.
(a) Each certificate holder shall provide at least the following flight attendants on each passenger-carrying airplane used:
(1) For airplanes having a maximum payload capacity of more than 7,500 pounds and having a seating capacity of more than 9 but less than 51 passengers—one flight attendant.
(2) For airplanes having a maximum payload capacity of 7,500 pounds or less and having a seating capacity of more than 19 but less than 51 passengers—one flight attendant.
(3) For airplanes having a seating capacity of more than 50 but less than 101 passengers—two flight attendants.
(4) For airplanes having a seating capacity of more than 100 passengers—two flight attendants plus one additional flight attendant for each unit (or part of a unit) of 50 passenger seats above a seating capacity of 100 passengers.