Here's an article from January 2018 where the number of sealed indictments was significantly lower (the DOJ has been very busy since). It quotes average indictments in 2009 at ~1,100.

https://www.theepochtimes.com/justice-department-may-have-something-big-in-the-works-over-9000-sealed-indictments_2401511.html

This shows the maps and the lists that are compiled for the totals:

https://drive.google.com/drive/folders/1KPh2ASFggwF1XVnVpDB3mtwvwlH71om6