how to properly use mysqli->prepare and bind_param

Question

how can I use the mysqli->prepare with the bind_param I am trying to log in using these two functions but I can think of the following error

Only variables should be passed by reference mysqli_num_rows() expects parameter 1 to be mysqli_result, object given

my code

$sql = parent::conectar()->prepare("SELECT id FROM admin WHERE email = ? AND senha = ?");
            $sql->bind_param('ss',$this->getEmail(),$this->getSenha());
            $sql->execute();
            parent::desconectar();


            if(mysqli_num_rows($sql) > 0){
                $aux = $sql->fetch_object();
                $_SESSION['idAdmin'] = $aux->id;
                $_SESSION['logadoAdmin'] = true;

                return true;
            }else{
                return false;
            }