From the Listing Counters (Admin Interface -> Settings -> Listing Settings -> Global Settings -> Listing Counters) menu enable:

Category Counters

If checked, allows the display of category counters on selected pages.

Note: Counters are updated using only the cron job. If they are out of sync, please use the initialization tool.

Hide Empty Categories / Options

If checked, will hide categories or filtering options that do not contain any listings.

Initialize Counters

If clicked, will start the process of initializing the site's category counters. Please wait until the completion confirmation message is displayed before navigating away from the page.