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This number is taken from anet own forum
As it was the preliminary and necesary step to download the client (barring the persons who used the unofficial torrent) , thats the closest number we are going to get to an official figure of beta event participants. Assuming 90% of participants employed the legit download and only 10 % the torrent (a conservative approach) that would put numbers of prepurchases around 220 k.
It could all be bogus, ofc.