Simple. I think Tesla current has at least 100 stores/service centers? 100 people X 2 reservations per person X 100 stores = 20,000.
I can't find the exact number of stores/galleries/service centers.
12,000 Tesla employees X 2 = 24,000
30,000 Deposits from Model S or X owners (Purely a guess)
50,000 (About 5%) of the 956K people who follow Tesla Motors on Twitter.
2,000 people at the unveiling
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= 126,000 Deposits
I realize this is not a statistically valid way to reach 100,000 + nonetheless it leaves plenty of room for error