14 CFR § 91.533 - Flight attendant requirements.
(1) For airplanes having more than 19 but less than 51 passengers on board, one flight attendant.
(2) For airplanes having more than 50 but less than 101 passengers on board, two flight attendants.
(3) For airplanes having more than 100 passengers on board, two flight attendants plus one additional flight attendant for each unit (or part of a unit) of 50 passengers above 100.
(b) No person may serve as a flight attendant on an airplane when required by paragraph (a) of this section unless that person has demonstrated to the pilot in command familiarity with the necessary functions to be performed in an emergency or a situation requiring emergency evacuation and is capable of using the emergency equipment installed on that airplane.