This is actually posted in the wrong section since you're asking about AACOMAS, but I'll answer anyways. There also isn't a designated activities section but it's divided into sub-categories.
There is no set number, high or low. This is a big plus because you can elaborate and include more into that you can in AMCAS. Don't include things that aren't meaningful. I included everything I would include on a resume:
- past work experience (if relevant)
- research experience
- publications, abstracts, conference posters
- collegiate athletics
- volunteering
- shadowing