Suddenly, the Apple Mail icon in the dock was not longer showing unread items as a number in white against a red background. I wonder whether I inadvertently turned something off.
To get it back, go to Apple / Notifications, and choose Apple menu > System Settings, then click Notifications in the sidebar.
Choose which mailboxes to include in the count of unread messages in the red circle, called a badge, that’s shown on the Mail icon in the Dock. In my case I want the unread messages from all the email accounts in Apple Mail.
Then activate the unread count, but note this important caveat. The count appears only if “Badge app icon” is selected for Mail in Notifications settings.