A nuclear arsenal that ensures mutual destruction is sufficient; having more is meaningless and would waste excessive resources on nuclear weapon maintenance. Russia currently spends over half of its military budget on nuclear weapon maintenance, contributing to its slow military development.
Thus, after ensuring the ability to achieve mutual destruction, the focus should be on building a comprehensive ballistic missile defense system. This will ensure your invincibility.
Complete Destruction of US and all its allies will likely require 5k plus warheads. So, China has long way to go before achieving that. Destroying one full city the size of new york will require atleast ten 500KT warheads. US has 346 cities with 100k population and most of these cities are much more spread out than other countries. It will take alot of warheads.
Even if most cities are destroyed, you still need devote warheads on military bases, missile silos, factories and so on. So, overall count of warheads needed to achieve full destruction is many thousands.
So, no, China with its current warhead and launch vehicle numbers, does not have the ability to ensure "Mutual Destruction". What they have is the ability to inflict severe/"unacceptable" damage on US.
Its because of this reason China is expanding its arsenal. I don't think China will stop until they get atleast 2-3 thousand warheads and even get to 5000 just to ensure parity with US and Russia.