You already gave the answer yourself. You see, ions don't change the number of protons and neutrons of an atom. It only changes the number of electrons. Ca originally has 20 electrons, protons, and neutrons. If it becomes ionic; two of the electrons disappear, but the amount of protons and neutrons are still the same. Now since the two electrons disappear, you place a "2" behind the letters "Ca". And now because the amount of negativily charged particles (electrons) are less than positively charged particles (protons), the atom becomes positive! Hence the plus sign after the number "2"? So the answer to your questions is 20. The only time the amount of protons and neutrons can change in an atom, is by the process of nuclear fission or fusion.