Obviously it's much harder to get statistics from pre-statistical societies, but there are some 'data points' going back to the Classical Era.
Imperial Rome maintained a standing army of 20 - 25 year veterans of about 500,000 men. This amounted to roughly 1% of an Empire population estimated (early 2nd century) at 60,000,000. This is less than 1/10 of what a modern state with 'universal' conscription can realize. BUT Rome's tax system was so inefficient that an army that size put immense stress on her resources, and after two massive (up to 30% deaths) plagues within 100 years her population couldn't support that sized army either - the mobile part of the Legions went from 5000 to about 1500 men, with the rest static militia manning forts along the border - and increasing amounts of the army were recruited from Non-Romans, including the German tribes the army was supposed to be fighting!
Tang China regularly sent armies of 70 - 80,000 men on campaign, but 90% or more of these were quickly-conscripted 'civilians in uniform' or porters carrying supplies. The principle assault force for one army of 80,000 was a 1000 man unit of mounted armored lancers. Those troops were so difficult to raise and maintain that the numbers simply could not be increased, while the losses among the conscripted peasants were so massive they spawned revolts. Given that the total population of the Chinese Empire at the time was at least 20 - 25,000,000, this negative result was caused by an even smaller percentage of the population than the Imperial Roman problems!
The War of the Spanish Succession (1702 - 1714) saw, basically, France against Europe - Britain, Netherlands, Austria, most of the smaller German states. France also had the major disadvantage that their opponents for most of the war had the best generals of the Era: John Churchill, the Duke of Marlborough, and Prinz Eugen - Eugene of Savoy. France mobilized up to 400,000 men in over 200 infantry regiments and close to 100 cavalry or dragoon regiments, by far the largest military force in Europe. This from a population of about 20,000,000 (Data Point: 19,700,000 in 1700), so still only about 20% of what France mobilized for, say, World War One in the Modern Conscript/Draft Army era. BUT by the last 3 - 4 years of the war, French infantry regiments were reduced to 1 - 2 battalions each instead of the normal 2 - 4, and French Cavalry Regiments, authorized 4 squadrons, rarely had more than 2. No amount of effort could raise more men out of the population which was largely sick and tired of the war. And by the end of the war, the French government was virtually bankrupt and unable to form any new military units or maintain the ones they had. Even Louis XIV, faced with this situation, admitted that "- Perhaps I have been too fond of War." - And this was a war that France is supposed to have won, in that they maintained a French candidate on the Spanish throne despite Europe's opposition.
What size military a state can support depends on much more than total population numbers. A big factor is what percentage of the population is required to maintain the food and production - pre-industrial agriculture sucked up at least 2/3 to 3/4 of the total population, leaving a much smaller percentage to carry weapons if you are also going to keep everyone fed - and alive: famine and plague regularly accompanied long wars up to the 18th century.
Another major factor is the efficiency with which the central government can get its hands on population, production, and gold resources. Rome's system of non-government 'tax farmers' meant the government spent very little on collecting taxes, but also received a tiny percentage of the taxes collected and so in the end could not support the military required to keep the Empire safe. Until states developed efficient bureaucracies, a large proportion of its Resources were simply not available to use for anything, military or civilian.