Kithens are usually located away from the terminal ( but not too far ), they drive the trucks to plane side and will try to load the galley before pax boarding ( ideally, but it doesn't always happen ). There is what's called " Stand-by food" at the terminal, which is a small kithen on site with prepared meals ready to board, and the person who is responsible for this flight will watch the load carefully, she/he will fill in more meals from stand by food as upgrades cleared. When the meals are overloaded, they will come on to take them off, too. When you are warned by CS about not having meals, usually it's when time is short and stand by food may not have enough time to bring the extra meals on.