CiviEvent: possible to trigger an email to event organizer when participant count reaches a certain number?
It can be easily done using hooks. Max participant is stored in civicrm_event table, so you need to fetch the value and then compare it with the count of participants registered in this event. All this check should be done after a participant is created. So, you could use post hook for the same. Sample(not tested) code is shown below -

function example_civicrm_post($op, $objectName, $objectId, &amp;$objectRef) {
  if ($op == 'create' &amp;&amp; $objectName == 'Participant') {
    //check the count of participant in the event
    $participantCount = civicrm_api3('Participant', 'getcount', [
      'event_id' =&gt; $objectRef-&gt;event_id,
    ]); 

    //Compare it with the max participant value 
    $max = civicrm_api3('Event', 'getvalue', [
      'return' =&gt; "max_participants",
      'id' =&gt; 1,
    ]);

    //If max equals the count, send an email
    if ($max == $participantCount) {
      //send email to xyz@example.com
    }
  }
}