Some WP themes show the post count for categories and archives by default, some don’t – to enable or disable them, go to wp-content/themes//sidebar.php

find the invocation for wp_list_categories and add (or edit) the following to the query string argument:
obviously, use 1 for ‘true’ and 0 for ‘false’

to show post count for archives, use:

note that these are different for the two signatures – show_count and show_post_count