How many gas are in the air? And.......?

Location is not a major determinant of the composition of air, unless you're also worrying about any local pollutants.

Air is about 78% nitrogen, and 21% oxygen, with the remaining 1% being mostly argon with tiny amounts of CO2, water vapor, and the remaining inert gases, plus traces of other gaseous chemicals.

The composition of the air is surprisingly uniform with no stratification of gases based on their densities. One of the properties of gases is that they will expand to fill their container. In this case the "container" is the atmosphere.

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Both Lucy and Magellean seem to have inflated the amount of CO2 in the air. CO2 is about 0.03%.
Take a look: http://www.synlube.com/images/Chemical_Composition_of_Air.JPG

how many you can identify depends on what is around locally and how precisely you are capable of detecting. In essence, all substances have a vapor pressure so there are molecules in the air from any nearby substances, even if the vapor pressure is extremely low and detection of those miniscule numbers of molecules is very difficult. In a way, that is the entire idea behind odors and our ability to detect smells.

Density can have some effect and produce stratification, but for the most part the gas phase is sufficiently mixed that such stratification does not occur except where there is a marked difference in gas densities and each of the compounds is present at very high concentrations (immiscibilty separation can occur or a source of one gas is discharging at a rate that exceeds the capacity of the system to fully achieve mixing).

Hi there. Interesting, but I think you are thinking of weight of the gases over their density.

The major gases in the atmosphere are Nitrogen (N2), Oxygen (O2) and carbon dioxide (CO2). There are other minor gases, like ozone (O3), argon (Ar), nitrous oxides (NOx), water (H2O) and lighter hydrocarbons. Carbon dioxide and oxygen are the heaviest of these gases, and so they are found in greater concentration closer to the surface of Earth than they are in the stratosphere. Oxygen is present in about 20% and nitrogen about 78%.

Hi :) Tbh it depends on where you are, but generally there is about 70% nitrogen, 20% oxygen, 2/3% Carbon dioxide, and the rest depend on where you are.
Hope this helps!
xoox