The bare minimum for participating is one member. However, for playing the game you need at least a driver, a human player, and if you need a secondary driver, then three people. If you have a mentor, then 4 people. Some teamshave anywhere from 4 people, upto 50 people. Sadly my team is about half a mile from the silicon valley regional, and yet we only have about 6 or 7 students plus 2 or 3 mentors.
A team from Hawaii came to the Silicon Valley regional a couple years ago and they came with 30 or 40 people, plus their 10 or so member pit crew. Your pit crew shouldn't consist of more than 5 or 6 students and a mentor. It gets to crowded after that, especially when local kids come down, and start going into the pit and getting in the way of everything. Ideally you want your 7 people tea plus a cheering section of about 15 to 30 people.