HOW TO get count of read emails in a Outlook.com folder

The new avatar of Hotmail, Outlook on the web shows count of unread emails but what if you wanted to know the count of all emails in a folder under Inbox (say for instance, in the folder Dev in the screenshot below)?

Select the folder and check the box next to the name of the folder. Click on the ellipsis in the top menu & select the "Mark as unread" option 

All the read emails will be marked as unread & the count of  emails will be displayed. Mark it as read after you finish noting down the number if you wish.