The principle is that, when you need MP to expand your military and recruit new units while at peace, you must extend the duration of the military service. Therefore, the actual growth of militarily available men in peacetime is proportional to the growth rate of the overall population – for the sake of simplicity we put it at the average, rounded value of world population growth between 19: 1% per year, divided between 365 days… a very low, but realistic growth, indeed. In any army, while young men are regularly conscripted, others complete their active service duty and return to civilian life. Please note that daily MP growth is now extremely slow in peacetime.
Every man fit for military service is mustered in, with teenagers of 15/16 and old men up to 60 or 65 also incorporated – often as a militia.