Is it possible to send only one issue (event) per email instead of combining multiple issues per email?

Yes, it is possible to only send one event per email. In the EventSentry console, scroll down the left side to the Actions section, and then select one of your email actions. On the right side, in the email action's settings, you can change the Events Per Email setting (in the bottom right corner) to "1" and repeat this change for all other email actions this setting should apply to.

After making the change push the configuration to all remote hosts.