Your flight crew depends on the size of the aircraft you are flying on and the scheduled flight time of your flight. The flight crew consists of the pilots (the cockpit crew), and the flight attendants (the cabin crew).
For United States registered airlines,the number of flight attendants required for each aircraft is dictated by U.S. Federal Aviation Administration (FAA) regulations. At the minimum, there must be one flight attendant per 50 seats. For example, a Boeing 737- 700 model aircraft can have 124 seats, therefore three flight attendants are required. As far as I know, this rule is also followed by airlines throughout the world, certainly by those that fly into and out of the U.S. Sometimes, depending on the airline, type of service (hot meals versus cold food), international versus domestic, and competition on that route, flight attendants might be added. The positions of the flight attendants also might vary depending on airline and aircraft.
For example, on our B737-700 aircraft with 124 seats, there will be three positions. The Flight Manager (FM) will be responsible to the Captain for all related passenger, service, and cabin concerns. S/he handles all necessary paperwork for such things as wheelchairs, special requests, oxygen, and unaccompanied minors (UMs) The FM usually does all of the public announcements. Another flight attendant will handle all First Class duties and be assisted by the FM, especially during boarding. The third flight attendant will be towards the back of the aircraft as you board and work mostly in the coach cabin during the flight. Of course, a good crew will work good together, assisting each other as need be and not just sticking to their assigned positions. On international flights, your lead flight attendant is called the International Service Manager (ISM) or Purser.
There always seem to be a lot of confusion as to what the pilot positions are, especially when the news media describes a crash or aircraft accidents and is writing about the flight crew. First of all, at least in the industry, all are pilots and no one is referred to as a "copilot". The Captain is usually the most senior pilot onboard and sits in the left seat as you face forward, the First Officer is in the right seat. These names are used for all purposes, from the actual flying and associated responsibilities, to administrative manners such as bidding for trips and vacation; and of course, pay. At most airlines, the Captain and First Officer alternate the takeoffs and landings.
Using our B737-700 example again, your cockpit crew will consist of two pilots. However, if your flight is scheduled more than 8 hours, a third pilot must be assigned. This is a U.S. Federal Aviation Regulation (FAR). This third pilot is usually called the International Relief Officer (IRO). The key word here is "scheduled" and can be a source of contention between the airline's management and the pilot's union. Additionally, there are other FARs that must be adhered to that also require an IRO. These are: in a two-piloted U.S. domestic series of flights, each pilot is limited to 30 hours in 7 days or international - each pilot is limited to 32 hours in 7 days unless an IRO is scheduled.
On ultra - long international flights there can be two sets of Captains' and First Officers' because of the very long scheduled time of 12 hours or more. Crew rest areas are provided for both the pilots and flight attendants on long international flights.