It comes out to about 1 Magus per 100k people. The 1:10k people is the rate of The Gift. Mark Shirley's numbers in Sub Rosa #14 made the mistake of ignoring the stated cannon rate of The Gift and set it at roughly 1:50k (1:49k if I remember right, Serfs Parma).
EDIT: Ack, population was a little over 60m in 1220. That would mean 1 Magus per 50k.