How to fix the wrong number of unread emails flag in Outlook?

Following works for me.

1. In the “Search Current Mailbox (Ctrl+E)” box, type: read:no and hit Enter.
2. When it shows “Find More on Server” link, click it. Then the unread email(s) should appear.

EDIT: Works with Outlook 2016 as well

you can see the image reference for your further help.

I've managed to fix this by going to the web access, right-clicking the problematic folders and selecting "mark all as read". After this, Outlook stopped displaying wrong unread messages count.

Run

outlook.exe /cleanviews

This cleans the internal viewstate and should resolve the problem.

WARNING: Running this command will also reset any custom views!