yourref.observeSingleEvent(of: .value, with: { (snapshot) in

  let value = snapshot.value as? NSDictionary
  let useremail = value?["Sender"] as? String ?? ""
  if useremail == "aa@bb.com"{
      //your local variable num
      num = num + value?["Number"] as? Int ?? 0
  }
  // ...
  }) { (error) in
  print(error.localizedDescription)
}