Action on every nth node creation
You could run an EntityFieldQuery on hook_node_insert to count all of the nodes of type event and then see if that count plus one (because the new node isn't saved to the DB yet) is divisible by five. Something like this:

function mymodule_node_insert() {
  $query = new EntityFieldQuery();
  $result = $query-&gt;entityCondition('entity_type', 'node')
    -&gt;entityCondition('bundle', 'event')
    -&gt;execute();
  $count = (count($result['node']) + 1);
  if (!($count % 5)) {
    // Generate/Send your email
  }
}


Note that the code above is totally untested.