This sorting order is available - and applicable - only for Newswire.

This is the most important metric for many News Agencies. When selecting Number of Matches as the sorting order of your stories, the system will show you Newswire stories that have accumulated the most matches. The sorting order is descending, which means you'll see the stories with most matches first. 

The number of matches appearing on every Newswire card, depend on your filter settings.

Example of a Newswire story. You can see the matches at the bottom right of the story card.

You can find this option at the Sort Ordering drop-down menu.