Question

why the amount of oxygen in the air is not depleted?​

Answer 1

The percentage of oxygen in the air does not change because the oxygen used in respiration and combustion is replaced by the oxygen generated by plants during photosynthesis.

Answer 2

Oxygen used up in some natural processes is renewed by another natural process. The percentage of oxygen in the air does not change because the oxygen used in respiration and combustion is replaced by the oxygen generated by plants during photosynthesis.