Soup will sometimes thicken up as it simmers depending on how thin the soup is.
However sometimes you need to add some cornstarch to the soup so that it can thicken faster as it simmers.
Also you can add rice to soup and allow the soup to simmer for 10 minutes or so and the rice will release starch into the soup which allows the soup to thicken.
If you want to make thick soup and not have the soup turn out thin you should always add a bit of cornstarch to the soup.
Adding cornstarch to the soup helps the soup thicken as it cooks so you don't turn out to have thinner soup.
After adding cornstarch to my soups the soups always turned out thick and not thin.
Although sometimes a nice thin soup is good to eat when you have the flu or cold virus or have a sore throat because the thinner soup is easier to eat and swallow than thicker soup.
But other times the soup is better when it's thick.