My guess is 6428963097 in first 24 hours.
I really? I think you're in error and have done extensive calculations to prove it. Globally Tesla currently has something in the order of 4000 people who could process payments in stores, plus their website, which can handle only ~30000 transactions per minute. The 4000 people will work Tesla hours (i.e. 24 hours without any break or slowing of pace, because everyone in the queue at closing time will be served), and will each process one order per minute. So, the 24 hour capacity for order will be as follows:
4000x1440= 5,760,000 reservations in stores
30,000x1440=
43,200,000 online reservations
Total 48,960,000
Thus you have overestimated the actual expected reservations by 6,380,003,097. I think I have spotted your error, in that you have overstated the website capacity. Thus your number is probably accurate for the first months reservations rather than the first 24 hours.
As an aside there are roughly 1.5 billion motor vehicles operating in the world today. The model 3 alone is set to decimate the global market while ending out with about a 97% global market share once Tesla completes the takeover of all productive worldwide industrial capacity and releases the already perfected, but not disclosed, propulsion system that uses NO2, O3, and CO as the active sources of power, while emitting pure water as an output. Thus Tesla will purify the planet earth too.
All this is happening because Elon now realizes that colonizing Mars is not practical in the short term so revitalizing the planet Earth is simply easier to do in the ten year horizon he has set.
Had you not made this miscalculation I would not have revealed this plan. The broad outline may be revealed in the Model 3 reveal, but I doubt he'll tell the whole story there for fear of almost instantly causing the Tesla stock price to rapidly rise to cause tesla to exceed Apple's market capitalization within a single month.