That's only like the active reserve force (The 500k) I belive China's actual reserve is way higher because every soldier that finishes their 2 year service recites a speech saying that they will return when war breaks out or something like that.
This, large portions of the population are trained due to selective mandatory service. Theoretically, calling up 5-10% of the population on short notice would be possible. But practically, it's hard to imagine for which scenario this would be needed.
But China as a rising power may need to attack others with Ground Force to attain its political goals. If China has to fight an Urban warfare in Taiwan for example. Street to street fight with just 1 million active ground force with such a low reserve could be problematic.
The armed police are very capable at counter insurgency. Their numbers can also be surged, like in Xinjiang.
If China had to deal with an internal uprising in Taiwan, armed police and a small number of elite PLA units with air/naval support would likely be the answer.
However, if Taiwan is attacked by large amounts of foreign troops, I.e. a real enemy invasion of China, then it is unlikely China would engage in protracted warfare with invaders and separatist fighters in the streets. Instead, they would cordon off the whole province by air, sea and internet, get the loyal Chinese citizens to vacate operation areas and enter refugee zones, destroy all infrastructure that isn't controlled the government forces, and then surround the surviving fighters from the countryside.
For that type of operation, they mostly need bombs and not too many ground soldiers.