There isn't a set number of super-curriculars that an applicant should be doing.
I don't think your work experience at the community centre is very relevant to your course, so try not put much emphasis on it on your PS.
I'm not applying for law but here are some of my suggestions.
You could:
- Find law firms to do work experience
- Keep up with the news, and maybe mention an article you found interesting in your PS? (e.g. a controversial recent case)
- Have you done any wider reading? There are reading lists published by universities that you can have a look at
- Are there any debating or law societies in your school?
- Participate in essay writing competitions
- Maybe you could watch a debate or visit the house of parliament?
- Undertake an EPQ or some form of project focusing on international law.
- Any subject related podcasts or magazines?
- Masterclasses, lectures etc.