I think my current team (#639) has about 30 students... We've grown more or less linearly from about 10 people since our first year. Also, I would say only about half are active each year. Further, I would say most of returning members who were NOT active the previous year make a lot of effort to be involved and helpful the next year.
Now, as for the largest team I know of, I know my team way back when I was in high school (#190) has had a ton of people in certain years. They tend to involve pretty much all of the Juniors on the FIRST team at some level... I think it might have been integrated into the curriculum since I was there. so that's 50 students right there. They actually require all Juniors to go to the New England regional

And then add to that the number of seniors that do FIRST for a second year. Also, there are college students at WPI that want to help out. I remember hearing that 120 college students had signed up for the team one year! Of course, only a small fraction of the college students can actually be useful, so I imagine the number must have shrunk to 10 or 20 at most. I am sure I've heard of larger teams... I just can't think of who they are.
I think the optimal number of students is a function of the resources available. If a large number of mentors are available AND a very good team structure exists, then a lot more students can participate and actually get something out of the program. If limited mentors or limited organization exists, a smaller number of students is the way to go. Typically, if there is room for involvement in the team, the students will come. If there is too many students and nobody can give them something to do, the students will go.... often with a time delay of about a year.
- Patrick
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Systems Engineer - Kiva Systems, Woburn MA
Alumni, Former Mechanical Team Leader - Cornell University Robocup - 1999, 2000, 2002, 2003 World Champions
Founder - Team 639 - Ithaca High School / Cornell University
Alumni - Team 190 - Mass Academy / WPI