How to show post count in category

I want to show the number of posts against each category in the main menu of my site. From my research I appear to have to use the line

<?php wp_list_categories(‘show_count=1’:wink:; ?> (changing what I think is a ‘0’ by default, to ‘1’:wink:

Problem is I can’t find this line in any of the files within the theme, so presumably will need to add the line somwhere, but to which file do I add it?