How many neutrons does isotope sodium-20 have?

""^20Na has nine neutrons............
By definition, $Z = 11$ for sodium. That is there are 11 protons, 11 massive, positively charged particles in EVERY sodium nucleus. If there are not 11 protons, then we have another nuclide. The most common isotope of sodium is ""^23Na. How many neutrons does this beast have?
""^20Na would be a short-lived isotope, with a half-life under a minute.