A long time ago we used to have a unit called the "gramme atom", the idea was that 1 gramme of hydrogen contained some huge but exact number of hydrogens, and that (say) carbon with an atomic weight about 12 times bigger would require 12 grammes to have the same number of carbons.
A mole uses much the same idea, but in a more modern and rigorous way.
Some gases, like hydrogen and oxygen are normally found as pairs in molecules. Single atoms of oxygen or hydrogen are very rare because they are extremely reactive.
How do you count atoms ?
Well, you catch one atom, write "1" on it...
Seriously, there is a rather neat trick....
It turns out that for most gases the same volume of gas contains about the same number of molecules, or atoms if it is a gas ike Helium which does not form molecules.
Thus 1 cubic metre of oxygen has close to the same number of molecules in it as the 1 of hydrogen, even though oxygen molecules are 16 times heavier.
The results of a chemical reaction are based mostly upon the number of atoms of each type involved, not their weight.
So we know that if we burn hydrogen in oxygen, twice the volume of H2 wil be used as O2.
A small pedantic point. MN doesn't use any sort of scientific notation, so Martianbishop means to say
6*10^23, not 6.022 x 1023
Ie
602,200,000,000,000,000,000,000
rather than
6022.2
So counitng them is more than a little tedious.