code: private function writeSupbookInternal()
      {
          $record = 0x01AE; // Record identifier
          $length = 0x0004; // Bytes to follow
  
          $header = pack('vv', $record, $length);
          $data = pack('vv', $this->spreadsheet->getSheetCount(), 0x0401);
  
          return $this->writeData($header . $data);
      }
docs: Write Internal SUPBOOK record.