A normal human breathes in and out about 8 litres per minute, and so the total needed per day is around 11,000 litres per day. However, of this total amount, around 20% breathed in is oxygen. Only 15% is breathed out, which means that 5% is absorbed by the human body then expelled as carbon dioxide.
This calculation is taking the human in an ordinary moment, whereas if he is exercising or overly stressed, the amount changes considerably. During exercise, he will need much more oxygen. Humans need oxygen to live, and the minimum oxygen concentration in the air for humans to be able to breathe normally, is 19.5 percent.