Do I need new mechanics/limitations to accomplish this? Nope, I just don't fill out 24 divisions ...
I just want to confirm what
@bitmode already said because even me that prefer more historical approach and less divisons per armies still cannot hesitate to fill up my armies to the fullest (max 24 divisons). My human nature automaticallly tells me "duh why not fill my army as much as I can".
However beside that ,as others said because General's upgrade works according the number of divisions this also force people to add as many units as they can under one command
Commanding several generals under 1 field marshal is annoying enough already... It makes orders a mess where they overlap into each other and just makes it pointless to make intricate battleplans...
Well ,Isn't this what the game is all for? let me remind people that thinks like you that the core of this game is about war and fighting
My emphasis is on Optionality ! so for those who are worried about the change ,then by making it optional you could still play with current limit and those like me who prefer more historicl approach can play with option that decreases the number of units under each command.
Regarding how authentic and historical the system is ,I'd like to remind you that germany used about 150+ divisons for Barbarossa. Now with the current system and Army Group can have 120 divisions. Now by having both upgraded FieldMarshal (who can have 6 armies) and upgraded general (who can have 30 Divisons) then an ArmyGroup can reach phenomenal number of 180 divisons

Players can easily cover the entire Russian front with just one Armygroup !!!