how can i get length of array attribute

how can i get length of array attribute, just like the code:
`{
    "name": "John",
    "age": 31,
    "city": "New York",
    "ls": [
        {
            "name": "John2",
            "age": "23"
        },
        {
            "name": "John3",
            "age": "24"
        }
    ]
}`
how can i get size of ls