You may adjust that by adding it to constants.php file:
If your web host or email provider cannot handle sending many emails at once, you can optionally add the following line to your constants.php to set a number to process per bulk mail cycle. In this example we are setting it to 25 per task cycle.
define ( 'BULK_MAILS_PER_CYCLE', 25 );
Use 100 or higher. Make sure that your hosting provider handle that large number mails per batch!