Just guessing but they have the 1086XXXXX and I see that is for some folks on March 3. I did it April 1 which is 1087XXXXX so I'm guessing they are doing it in 99,999 increments.
I also noticed that about an hour after my reservation was made, Musk was online saying they had reached 200,000. So the timing seemed to match that numbering system based on the stream of announcements on how many T3 reservations on the at first day, April 1.
Just a wild guess. A plug number to use in regard to discussion above on how the Federal Tax credit might play out.
Tesla faces tax credit puzzle