Adding a sump means more bio media becomes available and a higher turnover rate is made in your tank. That means you can keep more fish in your tank. As long as you don't overstock your tank by too much and have a reasonable schedule for water changes, you fan keep a lot more fish depending on how big the new sump is and how big your tank is.
So if you have a 100g tank and add a 20g sump, you won't be able to add many more monsters. But if you have a 300g and add an 80g sump, you could add a lot of other fish.