Get distinct count - column having array of array json value

Question

I have one table having json column named record we have values like

[ "billinfo": [
    {
      "sortKey": "z",
      "voucherList": [
        "021121",
        "021122",
        "021123"
      ]
    }
  ],
]


and 

[ "billinfo": [
    {
      "sortKey": "z",
      "voucherList": [
        "004077"
      ]
    },
    {
      "sortKey": "z",
      "voucherList": [
        "004076"
      ]
    }
  ]
]

Find Distinct Count on voucherList Ex, if there are six unique vouchers, return “6”