In WordPress 0.71, I modified b2-include/b2template.functions.php to show the post count when showing archive links (function get_archives), i.e. October 2003 (3).
In WP 0.72, there's a configuration setting called show_post_count that will do this for me. However, I don't see this option in the admin menus. I thought it would be under Other Options, as that's where the archive mode is set. Or am I missing something?