Creating a linkable category list with post count

by

  1. WordPress does this by itself as one of the widgets created. Just add the categories widget to a sidebar or wherever you want and presto, job done!

  2. If you need a more customized solution you’ll need to do a bit of coding.

what you want though is wp_list_categories() function.

<ul>
    <?php wp_list_categories( array(
        'orderby'    => 'name',
        'show_count' => true,  //adds count of posts per category
        'include'    => array( 10 )  //only includes category with ID 10
    ) ); ?> 
</ul>

see this link for reference: https://developer.wordpress.org/reference/functions/wp_list_categories/