The mole is used to describe practical-sized quantities of material in the macroscopic world. The Avogadro constant provides the link between this macroscopic world and the microscopic world of atoms and molecules. As a result of the definition, the mole contains a defined number of entities, usually atoms or molecules. This number is the numercial value of the Avogadro constant (𝑁A). (It would take you twenty thousand million million years to count up to this number if you were counting at one number per second).
The mole can be realised by certain very well controlled experimental methods known as 'primary methods'. One example is by weighing a chemical of known purity.
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More information on the SI unit definitions from BIPM