IIRC, the AA tour needs 10 people.

The ATC tours don't have a set number, I've done it solo, and I've done it in a group of 40 -- it all depends on their staffing. Once we have a reasonable idea as to how many people we have interested, I'll call the two ATC facilities and see what they could do for us.

As for the DFW tour, I have no idea what their requirement is, but I do know that it's run by the DFW Volunteer staff.