Power BI report server configuration

I can not give a formula but, my experience is, you don't need a high-end configuration for those given numbers. "Power BI report server" is just an IIS, but does not keep report files on file system. Instead, keeps them on report server DB. So SQL server which report db resides may be important, but again my experience is, it is not a big issue. What is important is your SQL server which holds the reports data (DWH/SSAS or whatever) and network connection between this SQL server and Report Server.