I, myself, would be happy to have a slew of recent cars come in. I prefer them and there are many from the last 10 years that I would like to see in Gran Turismo, some of which, frankly, are even glaring omissions.
Both the 200 and 300 numbers could happen. If GT Sport's content is carrying over, entirely, then the 200 mark has already been hit.
If it's 300, actually, then we have quite a number of cars to see. Polyphony is known for low-balling its car counts, in announcements, to only the amount they can guarantee.
Also, it's worth considering that, since this game will require an internet connection for most of its features, Polyphony isn't worried about having a rigid deadline for content, before disc printing. They'll work on things, like more cars, all the way to launch and you'll update when you connect to play. The car count will likely keep growing all the way to launch.