Cura/resources/definitions/custom.def.json

{
    "version": 2,
    "name": "Custom FFF printer",
    "inherits": "fdmprinter",
    "metadata":
    {
        "visible": true,
        "author": "Ultimaker",
        "manufacturer": "Custom",
        "file_formats": "text/x-gcode",
        "first_start_actions": [ "MachineSettingsAction" ],
        "has_materials": true,
        "machine_extruder_trains":
        {
            "0": "custom_extruder_1",
            "1": "custom_extruder_2",
            "2": "custom_extruder_3",
            "3": "custom_extruder_4",
            "4": "custom_extruder_5",
            "5": "custom_extruder_6",
            "6": "custom_extruder_7",
            "7": "custom_extruder_8"
        }
    }
}