If missing, the plate may be part of the main car_paint.dds / body.dds .
The introduction of Custom Shaders Patch (CSP) revolutionized number plates. Using Lua scripts, the game can generate number plate textures on the fly. assetto corsa number plates
Use Photoshop or GIMP with a DDS plugin to overwrite these textures with your own design. If missing, the plate may be part of the main car_paint