GetMaxVehicleID();

stock GetMaxVehicleID()
{
    for(new i = 1; i < MAX_VEHICLES; i ++)
    {
        if(GetVehicleModel(i) < 400 || GetVehicleModel(i) > 611)
        return i-1;
    }
    return MAX_VEHICLES;
}

if(strcmp(cmdtext, "/skokcarov", true) == 0)
{       
        format(string, sizeof(string), "Всего транспорта на сервере: %d.", GetMaxVehicleID());
        SendClientMessage(playerid, COLOR_WHITE, string);                       
        return true;
}