The record for the most simultaneous fire breathers (Around 82 I believe) at one spot was gained recently by Oceans of Fire II at Burning Man 2006
From the claim section of the Guinness site:
" Claim ID 125758 (Ocean of fire II) We filmed it!
At BurningMan 2006, 82 registered fire breathers gathered to set a new record for the most fire breathers gathered in one place to blow flames.
John Parulis of brightpathvideo.com filmed this event as the main cam. Video will be submitted to Guinness for verification.