What determines how many items make up a particular unit

The Avogadro's number determines how many items make up a particular unit, such that 1 mole of a substance contains 6.022 × 10^23 items.
These items may be particles such as molecules, atoms, ions , etc
Therefore, 1 mole of anything has 6.022×10^23 items.
This number is called the Avogadro's constant and applies to all substances, and is normally used by researchers and scientists to determine the number of moles of elements and compounds.

Cause its being chemically applied?