Question

How many people can breathe from one fully grown tree?

Answer 1

Trees provide Infinite amounts of oxygen which is sufficient for more than Lakhs of People to breathe
oundation, "a mature leafy tree produces as much oxygen in a season as 10 people inhale in a year." Here are some other quoted figures regarding the amount of oxygen produced by a tree:

"A single mature tree can absorb carbon dioxide at a rate of 48 lbs./year and release enough oxygen back into the atmosphere to support 2 human beings."​

Answer 2

a person breathes 9.5 tonnes of air in a year, but oxygen only makes up 23% of that air, by mass, and we only extract a little over a third  of the oxygen from each breath. that works out to a total about 740 kg of oxygen per year. which is why very,roughly, 7 or eight trees' worth.
hope this helps u