questo quello che ti serve

codice:
$sql = "
SELECT count(app_event.id) AS totaleid FROM app_event
JOIN app_event_book ON app_event_book.event_id = app_event.id
WHERE app_event.category_id='21'";
$row = mysqli_fetch_array($conn1->query($sql),MYSQLI_ASSOC);


echo $row['totaleid'];