I also agree they probably load a full meal spread, since the food is refrigerated and will hold even for a trans-oceanic flight. I have been on A320s where it was three of us in F and they had extra meals (though some were probably for the crew).

I have, however, heard of people upgrading at the last moment being warned that a meal might not be available, so if a flight is extremely light when catering leaves the kitchen (usually 60 mins before departure), they probably only load a few extra. So if the whole cabin suddenly upgrades out...