I don't understand your question, whether or not anyone has started looking at the 20 million cars by 2030 from a numbers perspective. That's the ONLY way I have ever looked at that goal, from a numbers perspective. And I think it a realistic and achievable goal. Whether Tesla achieves it or not is up to Tesla, no one else.
Elon is the one who made the goal, no doubt with consultation of others within the company who understood the challenges and opportunities. No doubt it's a monstrous goal but I think Tesla is very likely to achieve it, particularly if you give them a little wiggle room in terms of one or two years or a couple million in production. You see, Elon doesn't make such well-defined goals without considering how he will do it. Because, yes, that's an ambitious share of the auto market. But his plan makes sense. Elon realized, in order to have the least chance of failure, he needed to be the best manufacturer of autos. That's why he is so focused on operational efficiency and designing the cars to bring the best value to consumers.
Unfortunately, most people are lazy thinkers because they assume things they shouldn't assume. One of the most popular; that it costs a certain amount of money, plus or minus 5%, to make a car of a certain spec. Elon is constantly challenging every aspect of that assumption and proving it wrong. From the design and engineering of the cars they build to the way they are built and delivered. Elon wants to use smarter ways of doing everything to lower the price to the absolute minimum. He knows the theoretical floor is the cost of the raw materials. So he strives to create the same functionality with less material by using that material smarter. That's what's behind the use of gigacasting. It's also what's behind the constant effort to make the motors and aero more efficient. Because that requires less batteries which makes the car lighter and requires less batteries again. He even works on making the cost of the raw materials cost less.
I won't go into every way Elon and his teams are focused on making cars cost less because it covers every conceivable aspect of the process from reducing organizational/management deadweight, right down to the advertising and delivery. The combined efficiencies are headed towards being able to make and deliver the cars for half the cost of competitors. At that point, it's not a question of whether you can get 1/3 of the market, it's a given. Probably a lot more than a third of the market. A better car for less money. That's good for car buyers, new and used, and it's good for the economy and a car that costs less to build has a lower environmental impact. Even advertising dollars have a carbon footprint as does keeping acres of new cars in the sun waiting to find buyers. And Tesla's competition is really good at both of those things. Delivering real value to buyers? Not so much. That's why they are shrinking like Shrinky Dinks, and Tesla is growing like a wildfire in a dry pine forest. I expect that trend to continue because new car buyers like to get real value for their money and the competition is totally unequipped to deliver that value.
In my entire life, I have never witnessed a company perform at such an exceptional level. I wouldn't be surprised to see Tesla take over 50% of the market before 2036. Both Ford and GM achieved that at different points in history. This is what tends to happen when you think without artificial limitations.