Cura/resources/extruders/custom_extruder_6.def.json

{
    "version": 2,
    "name": "Extruder 6",
    "inherits": "fdmextruder",
    "metadata":
    {
        "machine": "custom",
        "position": "5"
    },
    "overrides":
    {
        "extruder_nr":
        {
            "default_value": 5,
            "maximum_value": "7"
        }
    }
}