I'd assume if you had a group of sorcerer-generals like the Nine, you'd be trying to keep as many of them 'in action' as possible- provided there were sufficiently trained mages around.
Not sure if the Scholomance was in any way aligned to the Light in TFL, but wouldn't they provide a steady supply of talented magic users if the Nine needed greater numbers?
But Pyro, your point is good. Just because they are The Nine doesn't mean there are Nine in service so far into the war. Perhaps some died earlier and have yet to be replaced.
LR
