But surely the same flight will be over/under-subscribed ocasionally, so the number of passengers should average 119 but occasionally 5 don't show up and sometimes 130 show up and have to be turned away. In the contract I don't think that the number of passengers should specified so much maybe a demand and the maximum passenger number of the aircraft could be stated.
For example Seats: 119; Demand: High will mean that on average the flight is fully booked but occasionally less so.
Alongside this the demand could change throughout the year. Not everybody wants to go to Gran Canaria in the summer when it's too hot or in the winter when it's too rainy.