Bu işini görür. Kendine göre uyarlarsın
<?php

     $q = $db->prepare("select * from kategori where parent_id=:p");
            $q->execute(array(
               ':p' =>0
            ));
            $say = $q->rowCount();
            
       
?>
<ul>
   <?php foreach ($q as $key => $value):?>
      <li>
      <?php  $subs = $db->prepare("select * from kategori where parent_id=:pId");
       $subs->execute(array(
         ':pId' =>$value["id"]
      ));
      $say = $subs->rowCount();
      
      ?>
         <a href=""><?=$value["kat_ad"];?>(<?=$say?>)</a>
         <?php  $sub = $db->prepare("select * from kategori where parent_id=:pId");
            $sub->execute(array(
               ':pId' =>$value["id"]
            ));
         ?>
         <ul>
            <?php foreach ($sub as $subKey => $subValue):?>
               <li>
                <a href=""> <?=$subValue["kat_ad"]?>(<?=$value["kat_ad"];?>)</a>
               </li>
            <?php endforeach;?>
         </ul>
      </li>
   <?php endforeach;?>
</ul>