20 million men at arms? That's one heckuva big Parliament.
China has many levels of "reserves" coming from every segment of life there. Think Airforce/Navy marksmanship training for basic recruits, now take away Airforce/Navy and replace it with "Orientation week at veterinary school" or some such thing. In theory service is mandatory, but they have a surplus of volunteers.
Maybe 10% of that 20 million number is anything close to what we'd consider "reserves". i.e. people the PLA could get into a gun, ammo, and a uniform, maybe some other gear. However, they have no way to get them anywhere, or supply them away from their homes for any length of time. The
active PLA is about 2.5 million. That's still a heck of a lot of Chinamen...
However that covers all branches of their military, Army, Air-Force, Navy, and Marines, and whatever other branches they see themselves as having.
In essence the "20 Million" number is a propaganda tool created by being very generous, counting every Chinese person who ever marched and carried a rifle. Maybe even the "young pioneers" shooting airguns at Mao-camp or whatever it's called there.
Then assuming something close to the usual 90/10% split between logistical functions and actual trigger-pullers, that puts them around 250,000 people serving as active combat-arms positions. Of course those are huge assumptions on my part, but I could be off by an order of magnitude and it's still way under the "20 Million" number people bandy about as being "China's Military".