There is a view in the ReportServer database call ExecutionLog3 (see https://docs.microsoft.com/en-us/sql/reporting-services/report-server/report-server-executionlog-and... ) The documentation is for SQL Server Reporting Services, but it is still valid for Power BI report Server. By default I think this view only holds the last 60 days of report executions (you can change the amount of days to keep in the properties of the Report Server when you connect using SSMS). So by default you will be able to see when a report was last run in the last 60 days and count the rows to see how often is was run.
We actually extract data out of that view and load it into our data warehouse so that we get a much longer view of the history of report executions without bloating the size of our report server database.
