Recipe calls for 1 can of soup plus 1/2 can water, but I have carton soup, how many cups does that equal?

Many canned soups are condensed (like virtually all Campbell's soups) and the liquid may be to thin it out. I'd be careful before substituting boxed soups, which are generally ready to serve.

Typically a tin of Campbell's soup is 10.5 oz.
You can google your specific brand to double check.
When I thin out a "cream" soup, I use skim milk instead of water.

A soup can contains about 11-14 ounces, depending on the brand. Sounds like the recipe is pretty forgiving if that's the way it's written.