I have read that the number is six plus strat. value of Army commander.
Mac commands the Army of the Potomac, but I can only create 2 AoP corps, the third corps and others become the corp of a nearby army (2 regions away).
I tested this on the standard 1862 full campaign scenario, ver 1.07f
I went to the AoP which starts with six corp. I removed them all as corp commanders and then reinstated them. The first two returned as AoP corp, the others became Army of Virginia corps.
I am probably dealing with something that's just effecting me but if someone could check this out, I would appreciate knowing.
Thanks
Joe