The information that displays when hovering over a target group in an email delivery configuration is tied directly to the Group Summary table on the group itself. For static groups, it's possible that the Group Summary table has not been updated since the group was created and therefore is displaying 0 rather than the most current membership count.  To generate the most current values, follow these steps:
  1. Go to Constituent 360 > Groups
  2. Search for the name of the group.
  3. Select View Members and verify that the group has members in it.
  4. Select the Group Information tab.
  5. Select the "Refresh Summary" link near the Group Summary table. Allow the task to run and populate the values.
  6. Return to your Target Groups selector in the email delivery configuration. The value is now populated correctly.