We would like to update you on a new feature added to the flight configuration which will allow you to set the Minimum passengers required for the first booking per flight, it is the same setting which you can find under the destination but in this case, you can do set it for a specific flight.
You can update it from the Flight Series screen using the 'Update minimum passenger' action:
Notice that the configuration in the destination as a higher priority, so if you have set 3 Min. Passenger in the destination configuration and 2 Min. Passenger in the flight, the system will still require the 3 passengers for the first booking.