EC's are only useful insofar as they help to demonstrate qualities such as team working, time management, leadership, communication skills, commitment, etc. If you do one thing which demonstrates all of these things well then you'll stand out much better than somebody who's taken 10 EC's and demonstrated them all poorly. Things like voluntary work (pro bono), mooting and executive positions with societies are all really good but there isn't a "magic number" of EC's which firms are looking for. Take on what you feel is manageable, aim to get the most out of it and you will really be fine.