Barley absorbs a ton of liquid while it's cooking, so the trick is not to be shy with the amount: I'd say for every liter of broth use about a cup of barley, maybe a bit more if you want it really thick. Let it cook for at least an hour and a half, two hours if you've got the time, because barley needs quite a bit of time to soften up properly and release the starch that's what makes the soup thicken. If it still comes out too watery, you can cook it uncovered for the last thirty minutes to reduce the broth, or just add a bit more barley halfway through cooking. The important thing is to taste as you go and adjust based on what you see, because it depends a lot on what type of barley you use and how much liquid your pot evaporates.