No certificate holder may operate an aircraft that has a passenger seating configuration, excluding any pilot seat, of more than 19 unless there is a flight attendant crewmember on board the aircraft.

FAA Link

Comments: Without exception any aircraft configured with a minimum of 20 pax seats must have a flight attendant onboard when flying on a Part 135 charter-certificated aircraft. Boeing's BBJ and Airbus' A319 are two of the newer aircraft on the market that meet this requirement although some air carriers circumvent the requirement by outfitting the aircraft with 19 or less seats.