Nicholas,
This, payload, fuel, aircraft weight etc, all goes on the loadsheet and from the loadsheet the crew know their performance figures pertinent to aircraft performance.
The airline, crew, will not know the definitive number of passengers until they have arrived at the airport and subsequently boarded the aircraft, even if we check-in online there is no guarantee we won't have a mishap on the way to the airport.
We see it on TV, even once checked in, a passenger may become drunk, develop a fear of flying or whatever, that's minus one male, one female, X number of bags or whatever to be deducted as last minute changes on the aircraft loadsheet.