It would take an infinite number, were the criteria that the parents be natural born. If the criteria is "native born" or "born a citizen" for the parents. Then it would take but 2 generations, that is the grandchildren of the naturalized citizens or citizens by virture of being on the winning side in the revolution.
Yes you are right, I made a mistake. should have been —
one born in the country, of parents who were citizens of that country.
Sorry for the confusion.