The Earth's atmosphere is about 300 miles (480 km) thick, but most of the atmosphere (about 80%) is within 10 miles (16 km) of the surface of the Earth.
Depends on your altitude. At sea level it's thick enough for people to breath easily. As you go higher it continually gets thinner. At the top of some mountains breathing is mildly difficult.
It depends on a lot of factors, including what you consider to be the atmosphere. For information, click on the link below.________The atmosphere does not end abruptly, like the surface of the ocean,...