[Jstylezzz]

something like this I guess

pawn Код:

new changedplates[MAX_VEHICLES];
public OnVehicleSpawn(vehicleid)
{
    if(changedplates[vehicleid] != 1)
    {
        SetVehicleNumberPlate(vehicleid,"YOUR_TEXT");
        SetVehicleToRespawn(vehicleid);
        changedplates[vehicleid] = 1;
    }
}

Or use Luis' code