How do i make a high poly car mod?

Link to tutorial?

Depends what kind of vertices he is referring to.

Mesh: Usually you don't count mesh complexity with vertices, but triangles. There's no limit I'm aware of. Our cars are on average 30-40k triangles with interiors. More triangles you have, more expensive to render, and potentially more spiky deformations if the physical skeleton isn't up to it.

Physics: Cars in BeamNG are made of nodes and beams (this is entirely different from the mesh. You don't take a car mesh and straight convert it into a skeleton). If he referred to the nodes are vertices, than that makes a bit more sense. There's a upper limit, but not one you can normally reach. Our cars usually have around 300-400 nodes. More nodes (and beams), the more to simulate, the heavier on performance. The amount of nodes usually depends on the complexity of the thing you want to have. If you are making a box, 8 nodes would do it, for example.

 

I don't see it mentioning any vertices limit, may you point the exact part you are referring to?

Also note that in the tutorial he is creating a simple vehicle, for simplicity sake.
Once you learn how to work with Nodes and Beams, you can re-use what you learned on a bigger scale project.

If you are approaching modding, I warmly recommend starting from something simpler. Making a vehicle is something I'd aim when you have enough confidence and knowledge about modding the game.

 

Please understand that a Jbeam is not your 3D model exported as Jbeam.
If you try to do that, you get a physical skeleton with very little use.

Jbeam is a different, ad-hoc structure you have to create to give the car it's physical behavior.
The 3D model is just for the visuals.

You need to make a Jbeam model from scratch, with physics in mind.
Load any official vehicle, and use the Debug menu to show it's skeleton (Nodes and Beams), and you will see how different that is from the 3D model itself.