A flight number can look like any of these:
* LH 702
* SR 3963
* BA 015
As you can see, there is no rule as to how many numbers there will be in a flight number, although it will always begin with a two letter code indicating the airline.
Some flight numbers have a logical relation:
There is a direct flight from Madrid to Moscow three times a week, the flight number is SU 300. The return flight number from Moscow back to Madrid is SU 299 (SU is the airline Aeroflot)
The flight number is shown on the ticket, on the flight departure boards at airports and all electronic and computerized information systems.
On some routes, there are so many flights each day that they can depart within a few minutes of each other.
Air France (AF) has over 20 flights a day from Paris to Nice. Some depart within 20 minutes of each other.
That's why it is important that the traveler has both the flight number and the departure time.