Size array in MongoDB

Question

I would like to see the size of the array in my document.
the document is this

{
    "_id" : ObjectId("5fad41da4157f326b6ab7b97"),
    "Nome" : "FARMACIA DUE TORRI",
    "Tipologia" : "Privata",
    "FAQ" : [
        {
            "testo" : "Bla bla bla",
            "tipologia" : "Prenotazione farmaco",
            "data" : "4/10/2020",
            "Risposta" : {
                "testo" : "Bla bla bla",
                "data" : "7/10/2020"
            }
        },
        {
            "testo" : "Bla bla bla",
            "tipologia" : "Orari apertura",
            "data" : "6/10/2020",
            "da Utente" : "Michele Bianchi",
            "Risposta" : {
                "testo" : "Bla bla bla",
                "data" : "8/10/2020"
            }
        },
        {
            "testo" : "Bla bla bla",
            "tipologia" : "Prenotazione farmaco",
            "data" : "12/10/2020",
            "da Utente" : "Valerio Arancio",
            "Risposta" : "Nessuna risposta"
        }
    ]
}

I would like see the size of FAQ, is an array thanks you so much.