The Dukes ordered 2,500 for the '02-'03 school year. We went to three events that year and two events last year plus we show our robot off to grade-school kids and we always take a bunch of buttons with us when we do that. We just got low enough on our supply that we purchased another 2,500.
btw - they cost us just over $600 shipped for 2500 2.25" buttons from
http://www.buttonsonline.com/.
Incidentally, if you look in the "Site Information" pages for each regional, you will see that buttons are the only items that you may
legally trade at events, although I've never seen this rule enforced at any regional I have attended.