14 CFR § 121.391 - Flight attendants.
(a) Except as specified in § 121.393 and § 121.394, each certificate holder must provide at least the following flight attendants on board each passenger-carrying airplane when passengers are on board:
(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.
(b) If, in conducting the emergency evacuation demonstration required under § 121.291 (a) or (b), the certificate holder used more flight attendants than is required under paragraph (a) of this section for the maximum seating capacity of the airplane used in the demonstration, he may not, thereafter, take off that airplane -
(1) In its maximum seating capacity configuration with fewer flight attendants than the number used during the emergency evacuation demonstration; or
(2) In any reduced seating capacity configuration with fewer flight attendants than the number required by paragraph (a) of this section for that seating capacity plus the number of flight attendants used during the emergency evacuation demonstration that were in excess of those required under paragraph (a) of this section.
(c) The number of flight attendants approved under paragraphs (a) and (b) of this section are set forth in the certificate holder's operations specifications.
(d) During takeoff and landing, flight attendants required by this section shall be located as near as practicable to required floor level exists and shall be uniformly distributed throughout the airplane in order to provide the most effective egress of passengers in event of an emergency evacuation. During taxi, flight attendants required by this section must remain at their duty stations with safety belts and shoulder harnesses fastened except to perform duties related to the safety of the airplane and its occupants.