You can put any amout of passengers you want but dont put them too low though......wont look nice =P i usually put it between 75% to 100%
You can see the amount of fuel used somewhere in the flight simulator.....cant remember where though....press alt at flight end (without exiting flight) and find a fuel option somewhere there and it should tell you the amout of fuel left....that with the amount of fuel you have at the start will tell you how much fuel you have used