14 CFR Part § 121.391.
It depends on the size/weight of the airplane and how many passengers it can hold
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:
- 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.
http://www.gpo.gov/fdsys/pkg/CFR-2011-title14-vol3/pdf/CFR-2011-title14-vol3-sec121-391.pdf
Parts § 121.393,394 relax these requirements when a plane stops, but denotes how close by the flight attendants must be, and other things like exit door requirements.
Remember, Flight Attendants are there for your safety :) which is the motivation for these regulations.