It can vary school to school. I was at a group interview yesterday where about 10 of us had to talk about two prompts given to us by professors, while they watched us. This is called a fish bowl. In another, about 16 or so of us had to interview one another and report back to the group about why the other person should be admitted to the program. In another, it was only about 6 of us talking to the professor and to one another about the program.
In any case, be aware that they are watching your social skills and how you come across to others. Don't be rude, don't interrupt (it can be hard since many people are trying to stand out at once & everyone wants to talk), don't monopolize, and don't be a silent bystander. Just be yourself, let your personality shine, & you'll be fine. Best of luck! 🙂