What are the breathing requirements of 9 B illion daily? What is the percentage of oxygen burned by gasoline?

There is no depletion of oxygen going on. The very small fraction of one percent of oxygen used up is only of concern for the CO2 produced. That is because CO2 is only 0.035% of the air, so a 0.01% increase is a big change. But oxygen is 21% of the air and a 0.01% reduction is no big deal. So the air is only 20.99% oxygen, no problem.