Best estimates are
2500 copies to GenCon, about 300 copies left over after GenCon. My guess is Paizo estimated a sale rate of about 100 copies an hour (the fire alarm late in the day on Saturday, an anecdotal accounts of a number of gamers deciding to head for dinner aftwords may have disrupted sales).
Ummm.
I don't think a fire alarm at 8:45pm would have disrupted sales.....