The amount of orzo varies depending on your preferences for pasta-to-broth ratio. For 16 cups (4 liters) of water, you use approximately 1 cup of dry pasta to make a thin soup to 2 cups of dry orzo to make a thick soup. The ratio I recommend is in the middle (1 ½ cups of dry orzo) so that if you need more liquid for your soup you can add hot water at the end.