Gran Turismo 3 supports only six cars. As I stated once, there were two sides to this when it comes to more cars. (1) More cars to a track are more tempting, at the expense of slower FPS rates. (2) Keeping 6 cars while making them accessible to 6 players via network, more cars would only taint the already overworked gameplay quality. Now let me choose a few games.
World Driving Championship (Nintendo 64)
Ridge Racer Type 4 (Playstation 1)
Ridge Racer V (Playstation 2)
Test Drive Le Mans (Playstation 1)
Le Mans 24 Hours (Playstation 2)
NASCAR Racing 2 (PC)
Formula One...(rest unknown, Nintendo 64)
Wipeout Fusion (Playstation 2)
Now, don't just pay attention to the ratings and styles of these games, I'm using them as references to prove a point. I am not a hardcore professional, but I noticed a few things about these games. These elements include graphics, frame rate, loading times, tracks and their quality, (optional) weather and realtime effects, and more.
World Driving Championship and RRT4 have up to eight for the win with ddecent graphic quality. Wipeout Fusion has weather effects on some tracks in this futuristic game. 16 to a track, but the game stays very fast with decent loading times and beautiful graphics. Ridge Racer V is one of the first PS2 titles, but even with its somewhat dated graphics, 14 racers compete for the win challenging each other in one massive city with massive routes. They've always done this in the Ridge Racer series. Most different tracks were RRT4, where there were four different areas. Note that I mentioned NASCAR Racing and Formula One. In this sense, this is okay. Because as I studied, the more cars look the same, the more machines can come to race. It's simple. Take one car model, slap on several paint schemes, boom! 22 cars for F1 and 39 or 43 for NASCAR. Now the game I mentioned the most. The Le Mans series marks the best efforts to capture sportscar racing at its finest. Keep in mind that many makes of car exist in these races. So more models of a certain type, the more loading times. In fact in "Le Mans 24 Hours", I can see why the other 12 to 24 cars don't get chrome mapping. It is because if everything was the same, gameplay would be same, but much slower. And don't expect loading times to get any better.
The point of any racing game is to make them fast so that a person can get in, race on, and keep a consistent frame rate. Anyone who's played GT3 realized that the game HARDLY EVER SLOWS DOWN. If GT3 wanted to go with 8 cars, that's still competitive. In fact, I'm surprised why 8 cars don't race at once for Sega GT, or any other 6-car-to-a-track game. If GT wanted more cars, maybe it can be relegated to same-car races (like Beetle Cup, for example). So my "expert analysis" is why 6 cars really isn't enough, but it is suitable so that an already fast game doesn't get stuck in 3rd gear, if you know what I mean. "Is 6 cars enough?" is not a bad question at all. I'm just going on logic and experience. Personally, I think PD can work with the team that made "Le Mans 24 Hours" and experiment making a multi-car racing GT series. Then the same team can learn how to make their respective games more accessible and have bigger packs to work with. Until then, six is all you're getting (not to sound or be mean).