I am in Fremont here. The Tesla manufacture site was Totota and GM's assembly factory Muni 10 before. I remember Toyota produced 300k or 500k corola per year.
I've read many comments like this, but I never find them very relevant. Not all cars are the same, and you can't produce very different cars on similar lines. More expensive or "premium" cars, as Tesla makes, have more parts, which means more floor space is needed for a production line, and producing cars with more parts means longer production times. Just because Toyota and/or GM produced XXX thousand cars per year at the NUMMI plant doesn't mean Tesla can or will.
In comments to IEVs News article, Nix (who nearly always knows what he's talking about) has claimed that the Fremont plant is undergoing a major expansion, and that there will be room for more production lines, and so Tesla will be able to make a lot more cars there. Perhaps... and perhaps not. I've read that Tesla has already had to increase the number of loading dock bays there, to handle the high volume of parts being shipped in.
One factory in an urban area can produce only a finite amount of product. No matter how much you expand the factory, the roads in and out of the place can only handle so much traffic, and the loading docks can only handle so much volume. This may not be so much of a problem in Nevada, at Gigafactory 1. With so much empty land around, they can always add more roads and more lanes to existing roads. Furthermore, with no buildings near Gigafactory 1, Tesla should easily be able to add lots more loading docks if it needs them. But in Fremont CA, I think Tesla is going to have an increasingly tight constraint on the logistics of having parts delivered and cars shipped out.
No auto maker of any real size tries to make all its cars at a single auto assembly plant. There are very real reasons for that, many practical constraints, and Tesla can't ignore those forever. Of course we know Tesla isn't trying to; it's already building a new assembly plant in Shanghai, China; and Tesla plans for a third somewhere in Europe.
Can Tesla build 500,000 cars per year at the Fremont plant, including all the Model S's, all the Model X's, and something in the neighborhood of 400,000 Model 3's? Well, I won't say it's impossible. Tesla has certainly surprised me before.
But my prediction is that the Fremont plant will max out total production (all three models combined) at less than 400,000 per year, and perhaps closer to 300k than 400k.
So it looks like Bob Wilson and I are on the same page on this question.