How many calories can the human body absorb in 24 hours?

Depends on how hard you work. Man hauling a sledge in the Antarctic, you'll need 4000 to 8 000 calories per day, and of that, at least 40% fat.
Ordinary folk get by with a few thousand calories per day, but if you are a blacksmith or a quarry worker, you might need 2000 to 4000 calories.
If you have less than 1500 cal you are unlikely to put on weight; less than 1000, you'll start to starve.
Your body is about 20% efficient in converting food calories into energy.
[As measured in rowing machine tests, where energy output and calorie consumption may easily be measured.]
Remember, the 'food calorie' is 1000 times that of the 'scientific calorie' which is the amount of heat needed to raise 1ml of water by 1oC.

NOTE: (added by bruce) It's my understanding that human body can absorb approximately 300 food-calories an hour or 7200 calories a day, assuming optimal conditions and variations among individuals, of course. Interesting tidbit; I have heard that athletes like M. Phelps, and riders in the Tour de France are incapable of restoring the calories they burn daily. They actually need time off to catch up. I'd be interested to know more about this myself.