Well, you can always divvy up jobs, such as car and track modeling, and add people to that. But let's do some quick math. Initially at least, it took six man-months to model one car. Let's say that towards the end it was much less, so that the average time is four man-months. If you put 100 of the team on car models, that still comes out to
(1000 cars x 4 months)/100 modelers = 3.33 years. And if the average is 5 man-months, it's over four years of work.
Just guessing, but that doesn't involve all the aspects of the car that might include damage and racing in the rain, sound and physics work. And it involves track models, game design, online coding, Home coding and all the other unknowns. Plus, it doesn't involve the massive interruption of GT Mobile that wasn't farmed out, probably on Kazunori's insistence. This game is his pride and joy after all.