Avogadro's number of molecules.
1 mole of any substance has Avogadro's number of molecules. 1 mole of gas, at a specific temperature and volume, occupies a predictable volume. At standard temperature and pressure (zero degrees Celcius and 1 atm.), 1 mole of an ideal gas occupies 22.4 liters and contain
This is Avogadro's number.