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.

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.