By default, when you open it, Outlook will show you how many unread messages you have in each folder by placing a number in (blue) after the file name as shown below.
We can change this so that instead Outlook displays the total number of items in the folder in [green].
To do so, simply right click on a folder and select Properties:
Change the radio control to 'Show total number of items' and click OK.
To undo this change simply follow the same procedure but select 'Show number of unread items' on the Properties window.