Probably London in November 2003
I don't have the exact numbers, but the London Do in November 2003 would be a top contender.
We did manage to fill up an entire pub -- the Windmill.
We also took up an entire floor of a Dim Sum restaurant.
And are YOU volunteering to organize the NEXT huge gathering of FTers? Be warned, it seems like it could be an overwhelming task -- like herding cats, as some FTers have said.