The Mobile Users by Client report displays the number of devices using Outlook and REST. More information on connecting via REST APIs can be found at this Technet article.
The Data Table
By default, the data table lists every device. You can filter it to display a specific Device Client Type by selecting the button under the graph. The table lists each user, their device, and their device Client Type. You can add, remove or re-order these columns to display the information in a customized way.
This report can be filtered, allowing you to generate bespoke reports about the devices in a particular Department, Country or any other category you choose.
As with all of the reports, this information can be customized, filtered and scheduled.