The Avogadro Constant, L is a constant number used to refer to atoms, molecules, ions and electrons. Its value is 6.023 x 1023 mol-1, like a dozen is 12 and a score is 20. Note the units of the Avogadro Constant.
For elements that consist of atoms (rather than molecules), the relative atomic mass expressed in grams contains the Avogadro Constant of atoms, whatever the element. Likewise, the relative molecular mass expressed in grams is the mass of the Avogadro Constant of molecules. As an example, 23 g of sodium contain the same number of atoms as there are molecules in 17 g of ammonia.
The value of the Avogadro constant can be determined experimentally.
See also The Mole | Avogadro's Law.