Avogadro's number:

The number of representative particles one of the substances known as Avogadro's number

The Avogadro's constant or number 6.02 x 10²³ is the number of atoms of carbon in exactly 12 g of Carbon-12.

Relation of Avogadro's number to the masses of chemical substances:

One mole of hydrogen atoms = 6.02 x 10²³ atoms of hydrogen (H) = 1 a.m.u

One more of hydrogen molecules = 6.02 x 10²³ molecules of hydrogen (H₂) = 2 a.m.u

One mole of water = 6.02 x 10²³ molecules of water (H₂O) = 18 a.m.u