Asuming
Assuming that SiegesNom holds the number of seats available before any bookings, you could:
Change the Record Source of frmBooking to
SELECT tblBooking.BookingID, tblBooking.PassengerID, tblBooking.BaggageNum, tblBooking.FlightID, tblAvion.SiegesNom, tblAvion.SiegesNom FROM tblAvion...