code: public function sendMultiple(array $messages)
      {
          $successCount = 0;
          foreach ($messages as $message) {
              if ($this->send($message)) {
                  $successCount++;
              }
          }
  
          return $successCount;
      }
docs: Sends multiple messages at once.
  
  The default implementation simply calls [[send()]] multiple times.
  Child classes may override this method to implement more efficient way of
  sending multiple messages.
  
  @param array $messages list of email messages, which should be sent.
  @return int number of messages that are successfully sent.