without a child theme (although this is not recommended), edit content.php in /template-parts/post/ and change this section:
echo '<div class="entry-meta">';
if ( is_single() ) :
twentyseventeen_posted_on();
else :
echo twentyseventeen_time_link();
twentyseventeen_edit_link();
endif;
echo '</div><!-- .entry-meta -->';
to:
echo '<div class="entry-meta">';
if ( is_single() ) :
twentyseventeen_posted_on();
echo ' - '; comments_popup_link();
else :
echo twentyseventeen_time_link();
echo ' - '; comments_popup_link();
twentyseventeen_edit_link();
endif;
echo '</div><!-- .entry-meta -->';
if using a child theme, create the folder /template-parts/post/ in the child theme, and add a copy of content.php into it;
the make the same changes as above…
https://codex.www.ads-software.com/Function_Reference/comments_popup_link