What is humid air filled of mostly?

Humid Air.

Humidity is the amount of water vapor (Water vapor is the same thing as water gas.) that is in the air. (When one speaks of how humid air is or the humidity in air, one does not count water in the form of droplets, such as the droplets in fog or in a cloud or in steam.)

We say the air is humid if there is a lot of water vapor in the air. When there is very little water vapor, then we say the air is dry or it is not humid or we say the humidity is low.

Dry air consists of approximately 78% nitrogen, 21% oxygen, 1% argon, .04% carbon dioxide. Besides those gasses, there is another .04% of hydrogen, methane, neon, krypton and other naturally occurring gases. (These are percent by volume, not by weight.) This is a remarkably uniform mixture the world over although the carbon dioxide content has been increasing.

The amount of water in the air varies greatly with the average being about 1% by volume. At a normal temperature, for instance 25 C (about 77 F) the air can not hold more than about 4%. (In a kilogram of air, at 25 C, the maximum amount of water is 2 grams.)

This means that there is always more nitrogen and oxygen in the air than there is water, even when it is raining.

There is no particular amount of water in the air that decides whether one says the air is humid or dry.

Because the amount of water vapor that can be in the air depends very much on temperature and pressure, one can not make a general statement about how much water is contained in humid air.

Humidity as a number:

To be very precise about the quantity of water in air one usually speak of absolute humidity and relative humidity. Absolute humidity is defined as the number of grams of water per cubic meter of air and that number typically varies from zero to about 30 grams. Relative humidity is defined as the percent of water in the air compared to the maximum amount of water that the air could contain at the same temperature and pressure. Thus, relative humidity varies from zero to 100 percent and what constitutes 100% varies a great deal with temperature and pressure.