Since soup is low in calories it can help fill your stomach up and make you feel full so you won't need to or want to eat other foods that are high in calories.
You can safely lose 1 to 2 pounds per week so by eating nothing but soup and exercising you should be able to lose at least 20 lbs in 10 to 20 weeks.
Losing anymore weight than that would be unhealthy but soup is good for you and fills you up while helping reduce the calories you intake and the less calories you consume the more weight you lose.