In my opinion the amount of fish your 12g can handle depends on the filtration and maintence you do on the tank. To me three SMALL fish in a 12g is fine adding two more clowns would probaly push it over the limit. If you did a 1 gallon water change every three days, I bet you could keep your nitrates, ammonia....etc. down. To me more importantly is the actual room for the fish, 5 fish in a 12g would be a little crowded considering how active clowns are. What are your other three fish? Also instead of adding any other fish you could think about adding a shrimp(s), they are also fun to watch.