In the process of rebuilding my little fantasy kingdom as a militaristic society, I've been looking at historical armies of the medieval era. I've been trying to work out how large of an army a given population could be expected to field. This is tricky since record-keeping was not terribly reliable in medieval Europe. So take all of this with a grain of salt.
Also bear in mind that who, exactly, served in the army varied a great deal. Military conscription of the sort that we currently use in the US/Europe dates from the late 18th century. Levies -- where all the able-bodied men of reasonable age were grabbed and handed some kind of weapon -- did happen, but these men didn't know much about fighting in formation and needed to be home to harvest the crops. Often, the military was a career path chosen by certain men and soldiers were professional full-timers. Their commanding officers would rent them out as mercenaries to (a) be able to pay them and (b) keep them busy.
I looked at the armies that France was fielding during the Hundred Years' War because they were fighting on home turf and they were a major player at the time. When you have to ship your men across a Channel and move through enemy territory, as England did, I would expect that to reduce the size of your army.
According to Fordham University, in 1340 France and the Low Countries had a population of about 19 million. (As an aside, Fordham has great online resources for medieval research.) At the battle of Crecy in 1346, France fielded 38,000-43,000 soldiers. Or more -- some sources put the army as big at 100k. Even on the low end, it looks like this was the biggest army that France sent against the British during that conflict.
38k is 0.2% of 19 million. Even 50k would only be 0.26%.
Out of curiosity, I tracked down some approximately contemporary Chinese numbers. The Yuan Dynasty (established by Kublai Khan) was in control of most of China and in 1290 the population was around 75 million (which was actually a big drop from the population under the previous Song Dynasty)
When the Yuan attempted to invade Vietnam in 1288, they sent 70,000 regular troops and 21,000 auxiliaries for a total of about 91,000. (This is a mind-bogglingly huge number for the timeperiod, too. They had trouble feeding them, apparently, which contributed to the failure of the invasion) That would be 0.12% of the population.
Right, in fantasy you get to cheat a bit. How much you cheat is up to you, but you should be aware that you're cheating. Armies march on their stomachs (as in, how well you can feed them). The baggage train is where all their stuff is. All those men are tired, hungry and bored so it's a gold mine for entrepreneurs who are willing to take the risk of catering to customers who are skilled with weapons. About the only thing that happens quickly among 20,000 men is catching something nasty when the guy next to you sneezes.
Plus, if all the men are out marching around in armor, who's working the fields? Sure, women can work fields but they also have children to take care of...
I'm going to be cheating and rationalizing it by saying that this is a society where serving in the military is close to a religious requirement. Everybody has a role in defending the kingdom, whether it's swinging a sword, driving a baggage wagon, or putting clothes on the soldiers' backs. My little kingdom has only about a million people in it, but they will be fielding a lot more than 0.2% of the population -- that would only be two thousand men.
Are you cheating? What's your explanation?