One could argue that multiple apprentices are already a necessity for the martial houses (Flambeau, Tytalus, etc). Take a look at this old thread on magi attrition rates:
https://forum.atlas-games.com/t/attrition-rates-for-magi/1311/1
I contend that since the martial house magi are almost certainly going to be lost at higher rates than other house magi, there is internal pressure to seek out and train more apprentices over the course of their lives. Perhaps not in groups, but if the average magus trains one or two over the course of his life, perhaps martial magi should have a goal of training two or three.
-DC