If you already have a sidebar displaying, then find the wp_list_categories by going to Appearance>Editor and going to the Sidebar.php file. Sometimes, it’s located on another file depending on the theme that you are using. Change the code to look something like this:

<?php wp_list_categories(‘title_li=&show_count=1′); ?>

Adding show_count to the arguments on wp_list_categories will display the total number of posts on your categories just like what you see on the right sidebar of this site.