How to represent sum/union types in in json schema

Use anyOf to assert that the property must conform to one or another schema.

{
    "type": "object",
    "properties": {
        "tracking_number": {
            "anyOf": [
                { "$ref": "#/definitions/tracking_number" },
                { "type": "array", "items": { "$ref": "#/definitions/tracking_number" }
            ]
    },
    "definitions": {
        "tracking_number": { "type": "integer" }
    }
}