Get length of children in Json

Get length of children in Json

Hi, I use Json for transform data from server to client here is my question:
How can I get length of Phones from PhoneBook...?
  1. {"PhoneBook": [
                {
                    "Phone": "9965212",
                    "Phone": "235645",
                    "Phone": "42155553"
                },{
                    "Phone": "2354444"
                },{
                    "Phone": "4512453",
                    "Phone": "54534232",
                    "Phone": "45687322",
  2.                 "Phone": "51231545",
  3.                 "Phone": "64353453"
                }
          ]
    }