How much chicken stock do you use to cook the pasta?

I probably use about 6 to 8 cups of broth when I'm boiling that much pasta. If you're short you can certainly make up the difference with water although it won't be as flavorful.

Since you boil the pasta in it, and then reserve 2 cups, I would guess that you need the same amount of broth as you would water if you were just boiling the pasta in water - probably at least 4 cups, if not a little more.