One way to solve this wold be to have 3 attributes on your entity.
1. AirlineName (String) → This can be set to whatever airline you wish to book flight for. You can set the value any number of ways
2. AutoNumber (AutoNumber) → This is self explanatory. Incremented number for each record created
3. FlightNumber (String) → You end up combing the first 2 attributes in this attribute. Something like this → $Entity/AirlineName + '-' + toString($Entity/AutoNumber) ….. So your end result is something like Delta-1, Delta-2, Delta-3, etc etc
Just one way of approaching this. Not entirely clear on what your use case is here but just something to get your wheels turning.
**You might also not need attribute #1 if you are selecting from an airline from a list. But again, just depends on your use case