Use the test feature of categories or segments to return list of contacts within the category or segment.
1. Go to Contacts dashboard
2. Click on Categories or Segments (left menu)
3. Search for your segment or category name
4. Hover over listed the name of segment (or category) and click on the down arrow seen on the Edit button (right)
5. Select the Test function from the drop down menu
A segment is dynamic and can change depending on your contacts (ie, unsubscribes). The number of contacts it returns may or may not match the number when tested at a different part of the day.