Cura/resources/extruders/custom_extruder_3.def.json

{
    "version": 2,
    "name": "Extruder 3",
    "inherits": "fdmextruder",
    "metadata": {
        "machine": "custom",
        "position": "2"
    },

    "overrides": {
        "extruder_nr": {
            "default_value": 2,
            "maximum_value": "7"
        }
    }
}