Unfortunately, none of our answers are correct. It's a problem
with Yahoo.
I emailed Yahoo Tech Support yesterday and received a reply
within an hour. It was their problem and they have found out what
caused it and repaired it.
Now my mailbox is back to normal. 
Great customer service.
Hello Ang,
Thank you for writing to Yahoo! Mail.
We welcome the opportunity to assist you with your concern
and
understand that your Yahoo! Mail Inbox is showing incorrect message
counter. We sincerely apologize for any inconvenience you may have
experienced.
Further, we would like to tell you that we've completed our
investigation and were able to identify and resolve the issue. Now
your Yahoo! Mail Inbox is showing correct message counter.
Please be sure to let us know if you have any further concerns.
Your satisfaction has always remained our primary concern.
It has been a pleasure assisting you. Have a nice day!
Be sure to check the Official Yahoo! Mail Blog for more information
and the latest updates about Yahoo! Mail!
- http://ymailupdates.com/blog/
Regards,
Yahoo! Customer Care