How many cores is BeamNG optimised for?

You put into words exactly what I was thinking... although I'll try to answer it anyways I guess.

My "guess" with the Ryzen 9 3900x is that it could run 30 cars about as well as my computer can... more than likely much better as I would imagine its single core performance probably far surpases my old machine. Remember, each vehicle basically exists in its own thread. So whether its a Pigeon or a Train, the entire physics simulation for it runs in just 1 single thread. That being said, naturally it will run better if the other thread on that core is left open.

So... basic rule of thumb... If its an 8 core processor, you will probably run 8 cars just fine. 12 core, 12 cars. 32 core... well... you get it. You can go further, but depending on the single core performance, more than likely you won't make it much more than 3 to 5 cars past your core count before the game starts running slower than real time.

HOWEVER... remember... the more cars you have on screen, the more draw calls that will be made to your graphics card. If you want to keep it above 60fps your not only going to need a hell of a CPU, but your going to need one HELL of a graphics card. (ok I make that sound like you need a super card or something... probably anything above a 1080 would get the job done.)