mylist.php

<?php

$children = $pages->get("/shop/")->children();
$list = new PageArray();

function mylist($children, $ind='', $arr){

  $ind .= "– ";

  foreach($children as $cat) {
    $countproducts = wire("pages")->find("template=product,categories=$cat")->count();
    $cat->title = $ind . $cat->title . " ($countproducts)";
    $arr->add($cat);
    if($cat->numChildren) {
      mylist($cat->children,$ind, $arr);
    }
  }
  return $arr;
}

$result = mylist($children, $ind='',$list);