Show count comments under the title

First, you should look at creating a child theme. That way, any modifications you make won’t be removed by a WordPress update.

Next, edit wp-content/themes/twentyseventeen/template-parts/post/content.php and look for the following code:

        if ( is_single() ) {
                the_title( '<h1 class="entry-title">', '</h1>' );
        } elseif ( is_front_page() && is_home() ) {
                the_title( '<h3 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark">', '</a></h3>' );
        } else {
                the_title( '<h2 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark">', '</a></h2>' );
        }

After that, simply add the following:

// Show number of comments
echo '<div>(' . comments_number( 'no responses', 'one response', '% responses' ) . ')</div>';

This should show an output of
Your Page Title
(13 responses)