Apple Mail shows a number next to the Spam mailbox to indicate how many unread messages it contains. Sometimes you’ll see the number, but when you click on the mailbox to see its contents you’ll see either no messages or no unread messages. This may be because:
The unread messages are actually in a second mailbox, inside the Spam mailbox. If there’s a triangle to the left of the mailbox icon, click to make it point down. This will reveal any mailboxes inside and allow you to access their contents.
Mail’s database is damaged so that the numeric display is out of sync with the actual mail that’s stored on your Mac and/or your server. To force Mail to rebuild its database, quit Mail. Then delete the file:
/Users/<username>/Library/Mail/Envelope Index
This file is called Envelope Index, and it’s stored in the Mail folder that’s inside the Library folder in your home folder. Mail stores the mailboxes and message data elsewhere, so they will not be affected. When you re-launch Mail, it will offer to import all your mail, i.e. rebuild its database.