About 15 per class. But you'll frequently be paired up with other classes in the first year. But honestly, in terms of social interaction, you'll be in contact with almost everyone within the study. It's common to have project members from another class, and it's actually recommended to chat and check out the projects from other classes!
Our study is a very tight community full of nerds, gamers and geeks, so it's very common to have a friend group that stretches over the entire study's generation.
Hopefully you'll find yourself in this community and study. Good luck!