Hey there! So the Roman Numerals refer to the oxidation state of the transition metals and not the number of atoms per molecule. Also, we don't use prefixes for ionic compounds since knowing the charges of the ions involved is sufficient info to assume the number of ions per molecule. I hope this helps!
would it be written out like: Co2(SO4)3 cross method Co^3(SO4)^2 that how we know its Cobolt (III) Sulfate?