Guessing he is including December estimate through today, since the question was about how many Model S have been sold "as of December 2015", with no specific date, and then you mentioned "100k by now." This, as opposed to December estimate through December 31.
Based on production numbers given by Tesla, and high Vin # of each quarter, I compute that from Q4 2013 to end of Q3 2015, about 7500 Vin #s were abandoned. I have the data if you need to see it.
The biggest gap were abandoned Vin #s assigned during Q3 and Q4 of 2014 as people switched from RWD to DWD models. Also, Vin # assignment is done by Tesla much earlier than other auto manufacturers. They apply a Vin # to your order long before a car is built (in some cases). Some may cancel an order or they may even decide to do "clean vins" at times.