Standard ratio for an industrialized power
How many people could US mobilize realistically?
How long could it sustain that mobilization?
How would it have to adapt to it?
Would they rather use nuclear weapons than do that?
Fit formilitary service:
60,620,143 males, age 18–49 (2010 est.),
59,401,941 females, age 18–49 (2010 est.)
Standard ratio in an industrialized power is 10 percent of the
total population can be diverted from the economy for military duty; obviously, this skews toward males in the age 20-30 cohort, but it gives you a ballpark. Obviously, one can only get those numbers from a population that will accept
conscription for the duration; all volunteer forces (India in WW II, for example) bring in so many complications that there is no ballpark.
More than 10 percent in a conscription regime, however, and labor force demographics for production take a hit; less than that, and the effectiveness of the field forces
should improve, given selectivity (fewer Cat IIIs and Cat IVs, to use the US terminology)...of course, the farther one goes in selectivity, the smaller the deployable field forces, which causes real problems in terms of generating combat power in the active theater(s).
Note that the above numbers do not differentiate between ground forces, maritime forces, and air forces; the 10 percent figure is simply a total. One can organize those numbers however one wishes. There is also some room around the edges for civilian specialist labor pools (merchant mariners, for example) that can be conscripted or organized into auxiliaries. Home defense forces (ADA, facility security, etc.) can also be part-time, if organized well, which fuzzes things out somewhat.
The US ratio for deployed ground combat forces in WW II (Army) was about 5-1; the Marines did better, because so much of their service force requirements were provided by the Army and Navy.
So, in a REALLY ballpark way, if one has a country of 100 million
that will accept conscription, one can estimate the numbers as 10 million in the military (i.e., outside of the production economy), of which presumably 2 million could be at the sharp end (ground forces). Given sufficient (~10 percent) cadre, a combat division filled through conscription can be overseas and ready for action within 18 months; less if the cadre is larger and selectivity is used for the fillers and replacements, especially if volunteers are called for (airborne, marines, etc.)
The above numbers are based on the US WW II experience, plus various studies at the ICAF; one final thing to keep in mind is the above numbers are just that, with no consideration of procurement, transportation, theater conditions, sustainment, etc.
In specific terms, the US is and has been closer to being an autarky (vertically and horizontally) than any other power in the world, for most of the past two centuries; whatever resources are not immediately available within the borders of the US can be (generally) replaced, stockpiled, or secured from (essentially) friendly neighbors. These include both natural, technical,
and human resources, which is not (generally) true for the other major powers. The US also has a superb defensible position, in relation to the other major powers (defined however one wishes) which all have in common their location in Eurasia, with less significant maritime frontiers.
Given the above, keep in mind that US practice and experience may not be entirely applicable to a Eurasian power engaged in mobilization without a maritime element. However, as essentially the only example of a modern nation state that engaged in total war but entirely at oceanic distances, the US practice makes some sense as a base case.
Best,