How Many Magi - https://forum.atlas-games.com/t/how-many-magi/7877/1
which refers to
Size and Shape of the Post-Schism Order - https://forum.atlas-games.com/t/size-and-shape-of-the-post-schism-order/5319/1
And I remembered a third one...
Magi Lifespans? - https://forum.atlas-games.com/t/magi-lifespans/6996/1
{using search-fu: "population"}
Add to this "Key Facts" in all 3 Houses of Hermes books.