General Staff supports 3 levels of command, e.g.:
Commander
Subcommander
Unit
Unit
So, it's up to you to decide what's appropriate for your scenario. So, in the OOB you described it may be:
Commander
Infantry Commander
Battalion 1
Battalion 2
Cavalry Commander
Squad 1
Squad 2
Do you have names and strengths down to the battalion and squadron level? Probably not. So you may opt for:
Commander
Division Commander
Infantry Regiment
Infantry Regiment
Cavalry Reigment
Technically, there isn't a physical limitation to the number of units but I wouldn't want to go over about 50. It would just get too big. A major feature of General Staff is the command structure. So you would order a cavalry regiment to charge and all subordinate units would obey (depending on their leadership, morale, etc.).