Based on some corporate filings...
tsla-10q_20160930.htm
Page 4
Customer deposits, under liabilities:
690,364 (thousands)
They had
$690 million in customer deposits as of September 30th, 2016. The most comparable figure, pre-Model 3 announcements, is
$283 million as of December 31st, 2015. (The March filing could not be used, because I'm not sure if it already includes some Model 3 reservations or if those hit the following statement.)
So there has been a gain of about $400 million during this time in deposits, which isn't far off from the original reservation count of 373,000. In addition, given the increase in sales (from $1.2 Billion per quarter in Q4 2015 to $2.3 Billion Q3 2016), there is likely some increase just due to increased deposits from customers buying the Model S or the Model X. The challenging part is that inventory sales are becoming a larger percentage of their transactions versus custom orders and as far as I know, we don't know that percentage.
Keeping it simple, if we assume that the original figure of $283 million had grown at the same pace as sales, deposits for the S/X were anywhere from $283 million to $548 million by the end of Q3 2016. (I assume no change in the percentage of customers that place deposits down for simplicity)
This means that Model 3 reservations are anywhere between 142,000 and 407,000. I think it's on the higher end, but not by too much more than the initial announcement. I wouldn't be surprised if we get more reservations after the final reveal (with option pricing, etc.) and that some were lost as they bought a Model S instead. Based on my assessment, these reservations are largely held by people excited by the technology. Most others who are less interested will wait for cars to be available at a dealership with a smaller wait time. At the same time, there could be some people who defer their reservations and so it's not shooting up as fast as I had personally expected.