I'd generally say that if you've put substantial effort into the EC and you can talk about it, then add it. Take a look at my re-app FAQ (link in my sig). I've got a post in there about listing ECs.
As Shifty said, quality over quantity. Many applicants pad these EC lists, and in the end that means nothing. Serious, significant, and unique ECs help an applicant stand out, even if there's few of them.
I also want to add that sometimes ECs can add a good deal to an application if they are relevant to your long-term goals. For example, if you have an interest in pediatrics and you volunteer with children regularly, that is relevant. Similarly, if you have an interest in sports medicine, ortho, etc, and you play a heavy-hitting spot, that is also relevant.