Well, the limit that a bunch of guys a while ago found was that 45 "things" could be spawned before the game crashed... granted that was like... 2 years ago now? Perhaps more? They were just spawning in cones at that time. You have sorred right past what they were capable of, so I am guessing the limit is somewhere in the hardware it is running on and not so much the actual game itself... just as Diamondback stated.
I strongly beleive it's in the game itself. I could hold over 20fps while moving my camera around with 65 cars in the game. But as soon as I add one, It simply shows a tiny no texture icon where the car should be. The object is still spawned and shows in the editor menu, but it is invisible to me. I'll try and get a screenshot of it.
There's a maximum node count limit iirc. The old "limits" were memory related, after a recent luajit upgrade that shoupd no longer be an issue. Try a car with fewer nodes, maybe you can get more to spawn
I just tested and I can have max 65 pianos, before it stops showing up ingame (and I have a beast of a pc)
So I'm not the only one. I litterly just did it with 65 trucks aswell, and got screenshots of what happens at 66.