Firebase Cloud Functions - numChildren()

I want to get the number of children of a snapshot.

Inside the receipts are different receipts:

/receipts/{receiptid}/ -> receiptid, user (...)

let ref = admin.database().ref(books/book/receipts/)
ref.once("value", function(snapshot){

//get number of children of snapshot
let number = snapshot.numChildren(); //DOESN´T WORK

});

via ktm125