Given China's large population they can bring an army of 200 million. However, that's just theory. No way a nation can sustain such a large army, 2/3 of US population, for a very long time. The logistics alone is almost impossible.

Furthermore, don't underestimate NATO in an all out war. The EU (almost all NATO members) has a population of over 500 million citizens and the US over 300 million - total over 800 million. Add to that the other nations that will side with the alliance (e.g. Australia, Japan) and you have a big population. However, nobody can field a 200 million army at once.