How do we find the roman numerals for Chemistry?

It's based off of the chemical formula. Example, Copper (II) Chloride is CuCl2, whereas Copper (IV) Chloride is CuCl4. The number represents the valence, or the electronic charge, of the atom. A good amount of elements are almost always the same valence, but transitional metals have multiple possible valences, so the roman numerals are there to clarify the valence.