Military sizes based on population
I have two countries that have been actively at war for 20 years, and I'm trying to figure out how big the general army should be realistically. I know during WW2 the US had up to about 12% of their population, with numbers dropping during other wars and during peacetime. For an intense war that is continually ongoing, what rough percent of the population would be in the military, including support services like postal and medics? I imagine it would change between the countries, kingdom 1, while focused on the war, would not be as invested as kingdom 2, which is in a dictatorship and heavily militarized. Any advice is appreciated 🙃
EDIT: kingdom 1 pop is around 25 mil, kingdom 2 is around 18 mil