If the reservation would be for free, I think we could easily see >500k reservations. Since it is 1000$, I think we can expect two groups of people reserve:
a) US residents who want to get in line before the 7500 federal tax credit expires - and here you really put your reservation down ASAP, otherwise I think it will be very tight and you may miss out - also a 1000 USD refundable reservation is a very reasonable thing to do in order to gain 7500 tax credit. I think this could be anywhere from 100k to 150k people for 2016. I expect the big wave of this come in early.
b) Die-hard Tesla fans who are - for whatever reason - just waiting for the Model 3. I don't think that's much more than 100k so early in the process. I see this as a much longer / more evenly drawn out process