Only one guest for per event attendee.

only_one_guest.html

<script type="text/javascript">
  // Limit event attendeees to one guest for an event.
  // @see https://help.salsalabs.com/hc/en-us/articles/114094526433
  $(document).ready(function() {
    if (RegExp('guestReg\\.sjs.event_KEY=999999').test(window.location.href)) {
      $('#addGuestButton').hide();
      $('#removeGuestButton').hide();
      var regheaderStrong = $('#regheader strong');
      regheaderStrong.html(regheaderStrong.html().replace('Register guests', 'You may register a guest'));
    }
  });
</script>