Under the theory of nuclear deterrence, most nuclear weapons states have little reason to build beyond that quantity of warheads. It's enough to effectively destroy any country. That's one reason why most nuclear weapons states stop near the 300 mark. The US and Russia are the exception because of their massive arms race during the Cold War.

Source: The Spread of Nuclear Weapons by Kenneth Waltz and Scott Sagan has a lot on this topic

Can you provide a link to the article and a source for the "unqualified assurance"?

First of all, I would like to note that the capability of available delivery systems matters almost as much, and perhaps more, than the total number of warheads. I can't really comment more on this topic in regards to the PLA 2nd Artillery Corps without violating the 20 year rule of this subreddit.

More importantly though, the PRC simply didn't have the military resources or technology to devote to a large scale nuclear weapons production program or large numbers of advanced delivery systems.

Lastly, the PRC didn't have a nuclear policy that relied upon being able to fight what could be a long, large scale nuclear conflict with the USSR or USA. Instead, the PRC appears to have adopted a nuclear policy similar to India or Pakistan: minimum credible deterrance Their nuclear arsenal is meant to make the potential costs of nuclear escalation with the PRC greater than the potential benefits of such a confrontation. The USA may be able to flatten every military base and major city in the PRC. How much are Americans really going to care though if the Chinese are able to hit say: Los Angeles, Houston, and Chicago with high yield warheads?