Here is a typical set of arguments included in the call to the wp_list_categories function:
wp_list_categories('show_count=1&title_li=<h2>Categories</h2>');
These arguments turn on the category post counts and display the name Categories above the list.
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To turn off the post count, remove the argument show_count=1 and any ampersand separating it from another argument. As an alternate, you could change the value to zero ( 0 ), which would have the same effect. If you want to turn the post count on, change the value to 1. If the show_count argument is missing, add the following as the first argument: show_count=1&. Do not forget to add the ampersand to separate arguments.