Most recipes will just call for a can of condensed soup, some recipes (or if you just want to make the soup by itself) call for plus 1 can of water. One can equals 1 1/4 cups of water. But as a healthier alternative to using water, try using whey if you recently made cottage cheese, or water used to boil potatoes. Milk can also be used in place of water to make a creamier soup.
Adapted from an internet recipe.