Politico put the number at about 1,500.
From the pictures Geller herself provides at the link above, my guess (based on taking some sample counts from various lengths-of-field and multiplying them out across like lengths-of-field sections) is that Politico lowballed the number, maybe by as much as 500-1,000.
By 38,500? Not a chance. The only way that crowd was over 2,500 is if several hundred attendees had midget barber shop quartets in their pockets.
I'm hoping Reuters/Fotoglif will turn up some more photos to make it easier; if so, I'll post one here [update: Got a Reuters/Fotoglif shot up now, but I've seen no "full-crowd" pics other than Geller's yet].