It's very difficult to say because some people like huge portions of soup while others like only a small amount. I personally always use the same pan to make my soups so I now know how far up the side of the pan the soup should be.
Another issue it's worth considering when making your own recipes is consistency. If you add a set amount of stock without considering how much you have of your other ingredients, you can easily end up with a soup which is too runny or too thick. To get round this, I would boil my vegetables (for example) in enough water that it just covers the vegetables and then crumble in two or three stock cubes to taste. It's also worth investing in cornflour until you have worked out the best way to get the thickness you want, as you're going to get a thicker soup if you are using root vegetables than if you were using tomatoes.