The numbers are usually 3 rotating dials, that can be changed by the captain or first officer to reflect the aircraft's flight number. If the flight is number 308, they would spin the numbers to reflect that
It would be printed on your boarding cards and/or ticket. It takes the form of an airline prefix followed by a flight number, for example; BA001 - British Airways flight 001 QF091 - Qantas flight 091...