Is there a way without haveing to use zmodeller like just editing the vehicle meta or cravariations to make the car accept for skins. For example i downloaded a cvpi with only 1 skin cvpi_skin_1 but i want it to have 3 more so cvpi_skin_2 and cvpi_skin_3. Is there a max number of skins a vehicle can have?

If they're multi-livery enabled by the modeler, then I don't think there's a limit, as long as you name the skins correctly in the .ytd file! :)

Also make sure that in vehicles.meta, the car shows in the flags "FLAG_HAS_LIVERY" (or something like that, it'll probably be in police3 and police lines anyway)

As long as the modeler enabled it when the released it it should be okay for multi livery