Think of every tt with an IPC value as having an infantry “depot”. The limit raised per turn based on the IPC value is reasonable.
But in the long term (and I’m thinking in particular of WWI) the number of men of fighting age available in any area is limited. WWI would not have gone on indefinitely; each side would simply have run out of men. Furthermore, the more you try to recruit in a particular area, the quality of troops raised decreases, and resistance to conscription grows.
Also consider depots as infantry training centres; a unit recruited there receives basic training, say the equivalent of a 1-1 infantry unit.
Leave it there another turn and it improves to a 1-2 unit, or you might use the turn to train it as storm troopers, paratroopers etc. You have to decide on the value of properly trained units against the immediate need for troops at the front, even if they’re just raw recruits to soak up hits.
A further refinement is to indicate the nationality of a depot; some nationalities will fight for more than one side, for example Romanian units might be raised by Germany or Russia depending on who controls Bucharest.
Units starting the game can be assumed to be fully trained, those raised from scratch start as raw. You can even consider some casualties to be wounded, and needing to be sent back to the depots for recuperation.