How can I add markup to a link in node links?
Unfortunately the object PluralTranslatableMarkup has no method to get the protected value count. The serialization was fixed to provide the count value, see issue https://www.drupal.org/node/2844181, but the class is still missing it's own getArguments() to extend it with the count value. You could file an issue and wait for a fix or use this work around with reflection:

  if ($title instanceof \Drupal\Core\StringTranslation\PluralTranslatableMarkup) {
    $reflection = new ReflectionClass($title);
    $property = $reflection-&gt;getProperty('count');
    $property-&gt;setAccessible(TRUE);
    $count = $property-&gt;getValue($title);
  }