How can I see the quota (number of participants which can still register) under the manage events section?
There is a BAO function in CiviCRM which returns the available slots for registration

CRM_Event_BAO_Participant::pendingToConfirmSpaces($eventId);


You can use api to get the max number of participant for an event

$result = civicrm_api3('Event', 'getvalue', [
  'return' =&gt; "max_participants",
  'id' =&gt; 1,
]);


To answer your question in detail, you canuse pagerun hook on event page and then get the variable assigned in smarty using $rows = $page-&gt;get_template_vars('rows'); override the variable to add or remove values and then assign it back to smarty using $page-&gt;assign('rows' $rows);

HTH

Pradeep