Nicholas,
Talking of me experiences in my younger days, and charter rather than scheduled airline, we (Ops) would receive passenger figures the evening before.
With B737/A320 and larger aircraft computerised flight plans need to be generated by Ops a few hours ahead of the flight, not more than a few hours before the flight because en-route winds can change affecting the flight time and fuel burn.
Airlines work on standard weights for males, females, children, infants and per bag, bags are weighed and often recorded but humans are not, the passenger figures passed the day before may only itemise adults (including children) plus infants, there may be no breakdown of males, females, children thus Ops may need to, on the flight planning stage, need to guesstimate this breakdown, some airlines may take worst case scenario and presume that all are male adults ..... the heaviest of the species
When the crew would subsequently report for duty they would be presented with the computerised flight plan, i.e. this is your routing and your payload is X amount of kilos, per kilo the aircraft needs to fuel accordingly, the heavier the payload the more fuel the aircraft will burn.
Prior to departure the handling staff present the crew with a loadsheet breaking down males, females etc, Ops may have presumed all male adults, the crew receive a lesser payload which, in theory, they've got more fuel than they require but, in one sense, one can never have too much fuel unless it affects the maximun landing weight of the aircraft at destination.
But excess fuel represents payload, it costs more fuel to carry that excess fuel/payload, I, personally, would monitor Ops estimates compared with actual payloads and, for the next flight to that same destination, perhaps reduce the weight per passenger by a kilo or so until more actual figures were being achieved for fuel planning purposes.
So, to answer your question, yes, the crew do know how many bags, they know how many males, females etc, if you watch these programmes on TV you might hear the term 'LMC' used, LMC is a 'last minute change' to the loadsheet whereas they may need to add one male and a bag, or whatever, and recalculate payload.
Hope this helps.