Avogadro never knew the Avogadro Number. To commemorate his contribution to the study of gases, Jean Perrin in 1909 defined the Avogadro Number as the number of molecules in 32 grams of oxygen - in the work:

Mouvement brownien et realité moléculaire. Ann. de Chim. et de Phys., 8. 1-114.

The name mole was proposed by Wilhelm Ostwald in 1893 in the book "Hand- und Hilfsbuch zur Ausführung Physiko-Chemischer Messungen" - "Nennen wir allgemein das Gewicht in Grammen, welches dem Molekulargewicht eines gegebenen Stoffes numerisch gleich ist, ein Mol".

Joseph Loschmidt made the first attempt to determine the number of particles in 1 cm^{3} of gas. In 1866 he published: Zur Grösse der Luftmoleküle, Sitzungsberichte der kaiserlichen Akademie der Wissenschaften Wien, 52, Abt. II, S. 395–413 (1866) - the value of this number was calculated on the basis of the kinetic theory of gases with an error of over 1000%.

The most accurate value was obtained from the X-Ray study of the silicon crystal - isotope 28.

On May 20, 2019, new definitions of the basic units of measurement of the SI System entered into force - the Avogadro number becomes an exact quantity. Depending on your needs, you can use the dimensionless Avogadro Number or the Avogadro Constant with the mol^{-1} dimension (1/mol)

Avogadro's number - 6.02214076·10^{23}

Avogadro constant - 6.02214076·10^{23} mol^{-1}