Good questions. According to this article, “the assumption that there are 4,000 cruise and ballistic missiles in the Russian inventory is not unreasonable.”

Context:

The Russian rate of production can only be estimated. Russia started missile production in 2015 in limited initial runs, and even in 2016 the production runs were estimated at 47 missiles. This means that it had only five to six years of full-scale production. The initial stockpile in February 2022 is unknown, but considering expenditures and the requirement to hold substantial stockpiles back in case of war with NATO, it is unlikely that the Russians are worried. In fact, they seem to have enough to expend operational-level cruise missiles on tactical targets. The assumption that there are 4,000 cruise and ballistic missiles in the Russian inventory is not unreasonable. This production will probably increase despite Western sanctions. In April, ODK Saturn, which makes Kalibr missile motors, announced an additional 500 job openings. This suggests that even in this field, the West only has parity with Russia.

As for the PLA, or for other estimates about Russia, I’d have to look around for other sources.

Also, I think this is an interesting passage regarding “flawed assumptions” about this issue:

The second crucial assumption is that industry can be turned on and off at will. This mode of thinking was imported from the business sector and has spread through US government culture. In the civilian sector, customers can increase or decrease their orders. The producer may be hurt by a drop in orders but rarely is that drop catastrophic because usually there are multiple consumers and losses can be spread among consumers. Unfortunately, this does not work for military purchases. There is only one customer in the US for artillery shells – the military. Once the orders drop off, the manufacturer must close production lines to cut costs to stay in business. Small businesses may close entirely. Generating new capacity is very challenging, especially as there is so little manufacturing capacity left to draw skilled workers from. This is especially challenging because many older armament production systems are labour intensive to the point where they are practically built by hand, and it takes a long time to train a new workforce. The supply chain issues are also problematic because subcomponents may be produced by a subcontractor who either goes out of business, with loss of orders or retools for other customers or who relies on parts from overseas, possibly from a hostile country.

PLA has been constantly modernising for the last 15 years or so, so i would assume their resupply capabilities are superior to any other country even now. And contrary to the west, they have centrally planned economy and world’s biggest industrial production, so they can increase the production much more and much easier than west.