No, you do not have to use Evaporated Milk.  The main reason I use evaporated milk in my white chicken chili recipe is since evaporated milk has half its water removed, it is thicker and creamier than milk AND it is less likely to curdle than milk.  That being said, you can substitute half n half or milk in this recipe if you'd like.  The flavor might be slightly less rich, you can fix that by adding a spoonful of better than bullion chicken base or maybe a bit more sour cream.)  For best substitution results, use 3/4 cup of whole milk and 1/4 cup of half n half for every cup of evaporated milk needed.