Outlook doesn’t have an easy method for counting the results of a search. There are scripts you can write to do it, but that’s not very helpful for your end users. Fortunately, you can use custom Search Folders to save a search query and get a count of the results.

These instructions are from Outlook 2016, but it should be very similar in previous versions of Outlook.

1. On the ribbon on the FOLDER tab, click New Search Folder.
   
2. Select “Create a custom Search Folder” from the “Select a Search Folder” box.
   
3. Select “Choose” to give your search folder a name and to specify the mailbox folders you want to include in your search results.
4. Click the Criteria button and define your search criteria. In this case, I used the Advanced tab to search for all emails sent within a date range.
   
   For the “field” I selected Field > Date/Time Fields > Sent. For the Condition field, I chose “between”, and for the Value field, I entered “10/1/2016 and 10/31/2016”. Don’t forget to click “Add to list” to save your search criteria.
5. Click OK three or twenty times and Outlook will save your new search folder and open it. The item count will be shown at the bottom of the window. If you don’t see it, right-click on the bottom left corner of Outlook’s status bar and select “Items in view”.

[Updated 10/24/2017/]