How can I provide my own replacement patterns to be used in config form?

I want to give a configuration form where user can configuration text on a widget, like the description and button label, however I want to provide a replacement pattern to be used, like {{count}} so the user can write for instance "{{ count }} images remaining". How can I provide replacement patterns to be used in the configuration form?

{{ count }} is a Twig variable pattern. Parse the content before you display it as Twig inline template, to replace the variable dynamically:

use Drupal\Component\Utility\Xss;

  $build = [
    '#type' => 'inline_template',
    '#template' => $text,
    '#context' => [
      'count' => $dynamic_count_value,
    ],
    '#post_render' => [
      function ($children, $elements) {
        return Xss::filterAdmin($children);
      },
    ],
  ];

Xss:filterAdmin() is only safe for admin forms. So don't make the config form accessible to other users.