The average pair of human lungs can hold about 6 liters of air, but during normal respiration, only about 500 mL (one half of a liter) is inspired.
Oxygen makes up 21% of air.
There are 2.7 x 1019 (2,751.3) molecules in every cubic centimeter of air.
0.5 liter (500 cubic centimeter) contains 1,375,650 molecules, 21% of which are 288,886.5 oxygen molecules.
(see Composition of Air)