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.