What species of Clownfish do you have? This will make a difference as some can grow quite large. For instance, if you have a pair of Maroon, Clarkii or Tomato Clownfish, I'd say you're at your bio-limit. If you have Percula, Occellaris or some of the smaller skunk species, you should be fine adding another fish. I wouldn't go with more than 1 more though.