In the grand scheme of Assetto Corsa modding, number plates might seem trivial compared to building a physics engine or 3D modeling a cockpit. But they are the final 5% of polish that separates a "video game car" from your car.
For a unique design, you can manually edit the car's plate textures. These are typically stored as plate_d.dds (the visible design) and plate_nm.dds (the normal map for 3D indentation) within the car's skin folder. assetto corsa number plates