Whilst googling for, um, something else, I stumbled across a report of the SuperNationals III, held in Nashville in 2005.
(this is a PDF document, not a normal HTML page).
The total number of participants was 5270 - an astonishing number! And these were all children so there must have been loads of parents, teachers, younger siblings etc. there as well.
Did anyone play in this tournament? I'd love to know what it was like playing in such a huge event. I went to a tournament in the UK a few months ago in which 1000 children played, and that seemed massive!