China hasn't agreed to any arms inspections, so the size of their nuclear arsenal is pretty vague. (Inspections are typically the result of bilateral arms reductions-- China doesn't have enough nukes to require reducing them, so there's no reason for them to let foreigners into their ICBM silos)
These are only estimates from our intelligence community, but they are considered relatively reliable. China hasn't built up a massive nuke stockpile yet, and its probably not within their best interests to do so (expensive, increased tensions). They could not hope to match the US or Russia today, so why bother trying?
China will continue to build up its conventional forces to project power in the region, that is probably more of a worry for the US than anything else.
