How to generate sites configured for external sharing report?
Perform the following steps to generate Sites Configured for External Sharing Report:
Select SharePoint Online Reports from the available vertical tabs. Then from left side of the application, select External Sharing Report.
On the selection of "Sites Configured for External Sharing" from the list of available reports, the resultant data is displayed as shown below:
 

     
Number of columns to be displayed on the Grid can be selected from the dropdown. By default, all columns are displayed.