I don't know, hard to say. I know DaveT predicts a pretty agressive ramp in demand for Gen3. I won't mind if he turns out to be right, and there is certainly room to grow, share to steal from the BMW 3, the Audi A3, etc.... but, what we have seen over the past couple of decades was a concentration of car brands into a few dozen big groups. BMW may be the only exception that can be profitable in the long run having their feet in only 1 market segment. Audi is part of VW, who make everything from Skodas to Lambos, from Porches to MAN trucks, and generally speaking all big brands try to offer a range of cars from cheap affordable to luxury. Even Mercedes tried that with the Chrysler marriage.
I wouldn't mind Tesla becoming a Tesla Group and selling their own affordable range under a sub-brand or even venturing into, or co-creating drivetrains for trucks and buses once the battery tech permits in, say, a decade. After all, even if others play catch up well, they may keep a technological leadership, an edge for a longer period if they play their hand right. If anyone, they should be able to produce 15-20k EVs first.
Having said that, with 100-150k demand for S/X (i do expect X to become stronger than S), and 350-500k Gen3 it wouldn't take much to get there. I think the Gern3 SUV has the possibility to be at least as big as Gen 3 alone (that small SUV class is all the rage now...), and if they release a new roadster on that platform, we should be way above 1 mill. Maybe in as little as the next 6-8 years if that small crossover comes by 2020.
The more interesting question is, how could Tesla turn into the 10-15 million juggernaut it could become if it keeps that edge, that leadership? 1 luxury brand won't be enough for that.