Modify Drupal pagination to restrict shown page numbers on Drupal 8
It would seem that at the point you're trying to modify the quantity, the pager items have already been generated so your quantity value doesn't have any effect. 

Similar to Drupal 7, I believe you're going to need to implement the full pager function, just copying the core pager function to your theme and changing the quantity. 

function MYTHEME_preprocess_pager(&amp;$variables) {
  $element = $variables['element'];
  $parameters = $variables['parameters'];
  $quantity = 5; // hardcode your quantity here
  global $pager_page_array, $pager_total;

  // Nothing to do if there is only one page.
  if ($pager_total[$element] &lt;= 1) {
    return;
  }

  $tags = $variables['tags'];

  // Calculate various markers within this pager piece:
  // Middle is used to "center" pages around the current page.
  $pager_middle = ceil($quantity / 2);
  // current is the page we are currently paged to
  $pager_current = $pager_page_array[$element] + 1;
  // first is the first page listed by this pager piece (re quantity)
  $pager_first = $pager_current - $pager_middle + 1;
  // last is the last page listed by this pager piece (re quantity)
  $pager_last = $pager_current + $quantity - $pager_middle;
  // max is the maximum page number
  $pager_max = $pager_total[$element];
  // End of marker calculations.

  // Prepare for generation loop.
  $i = $pager_first;
  if ($pager_last &gt; $pager_max) {
    // Adjust "center" if at end of query.
    $i = $i + ($pager_max - $pager_last);
    $pager_last = $pager_max;
  }
  if ($i &lt;= 0) {
    // Adjust "center" if at start of query.
    $pager_last = $pager_last + (1 - $i);
    $i = 1;
  }
  // End of generation loop preparation.

  // Create the "first" and "previous" links if we are not on the first page.
  if ($pager_page_array[$element] &gt; 0) {
    $items['first'] = array();
    $options = array(
      'query' =&gt; pager_query_add_page($parameters, $element, 0),
    );
    $items['first']['href'] = \Drupal::url('&lt;current&gt;', [], $options);
    if (isset($tags[0])) {
      $items['first']['text'] = $tags[0];
    }

    $items['previous'] = array();
    $options = array(
      'query' =&gt; pager_query_add_page($parameters, $element, $pager_page_array[$element] - 1),
    );
    $items['previous']['href'] = \Drupal::url('&lt;current&gt;', [], $options);
    if (isset($tags[1])) {
      $items['previous']['text'] = $tags[1];
    }
  }

  if ($i != $pager_max) {
    // Add an ellipsis if there are further previous pages.
    if ($i &gt; 1) {
      $variables['ellipses']['previous'] = TRUE;
    }
    // Now generate the actual pager piece.
    for (; $i &lt;= $pager_last &amp;&amp; $i &lt;= $pager_max; $i++) {
      $options = array(
        'query' =&gt; pager_query_add_page($parameters, $element, $i - 1),
      );
      $items['pages'][$i]['href'] = \Drupal::url('&lt;current&gt;', [], $options);
      if ($i == $pager_current) {
        $variables['current'] = $i;
      }
    }
    // Add an ellipsis if there are further next pages.
    if ($i &lt; $pager_max) {
      $variables['ellipses']['next'] = TRUE;
    }
  }

  // Create the "next" and "last" links if we are not on the last page.
  if ($pager_page_array[$element] &lt; ($pager_max - 1)) {
    $items['next'] = array();
    $options = array(
      'query' =&gt; pager_query_add_page($parameters, $element, $pager_page_array[$element] + 1),
    );
    $items['next']['href'] = \Drupal::url('&lt;current&gt;', [], $options);
    if (isset($tags[3])) {
      $items['next']['text'] = $tags[3];
    }

    $items['last'] = array();
    $options = array(
      'query' =&gt; pager_query_add_page($parameters, $element, $pager_max - 1),
    );
    $items['last']['href'] = \Drupal::url('&lt;current&gt;', [], $options);
    if (isset($tags[4])) {
      $items['last']['text'] = $tags[4];
    }
  }

  $variables['items'] = $items;
}