Which part of a chemical formula indicates the number of copies of molecules or compounds?

That would be the super script, which is the number that appears in front of the molecules composition. 
Ex. 3H2O: The 3 is the super script, and shows that there is 3 water molecules

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.