You welcome, I think
I can't of course say anything surely but I have always imagined that we can create on Theatre for anything we want to.
As Germany
One for Poland
One for Fall Gelb
One for Seelöwe
One for Mediterranian
One for Africa
Three for Barbarossa and one more to Finland
Et cetera.
Actually you probably only want one Theatre HQ facing the SU. Otherwise the AI would not be able to balance the front, respond to enemy troop movements and use reserves properly. The 'Nord' 'Mitte' and 'Süd' Armies can easily be subordinated to a Theatre HQ, or so I imagine. Apparently size does not matter, at least not concerning # of divisions (or army corps) below Theatre HQs in HoI3.