Twelve? Unless they want to increase their Navy budget and manpower threefold, that is unfeasible.
Five carriers including the Varyag is far more than enough for China to defend her territory, defend her shipping lanes, as well as to project power.
Building twelve carriers, especially if they are only from a single or a couple of classes, will take a long time, and when that's finished, they would already be obsolete as compared to US Navy aircraft carriers.
China could probaly build these in the space if 15 years if she really wanted to. It is all a matter of wish more than anything. A country that can pump out two 052C+ destroyers a year can quite easily build 4 aircraft carriers over 5 years if the military necessity is present.
I am not quite sure why you think the carriers will become obsolette. Most of the military power of an aircraft carrier battle group comes from the escorts, aircraft, and the radar and electronic systems that the aircraft carrier has onboard rather than the carrier design itself. After all, there is little difference in capability between a Nimitz and Ford class carrier battle group.