Purely technically, China have 7 ship yards with a 300m+ long yards:
Dalian (CSIC), Qingdao (CSIC), Huludao (CSIC), Shanghai (CSSC), and Guangzhou (CSSC)
so they can technically build more than 2 carriers at the same time.
Some of those yards have several slip ways longer than 300 meters, so by that logic they can simultaneously build 4 or 5 carriers in just one yard.
But it takes more than slip way 300 meters long to build an aircraft carrier. There needs to be a whole highly specialized industry with enough capacity to able to turn out power plants, fittings, and equipment at a rate sufficient to support normal rate of construction progress. The yard must have enough skilled shipyard workers to undertake the hull construction of large warships, which is much more challenging than building a 300 meter container ship or cruise liner.
Finally, if they build seven carriers at once, they will need an military aircraft industry able to turn out enough planes, plus attrition reserve, training back up, etc at a rate sufficient to equip these carriers when they are built. If it takes them 6 years to build each carrier, simultaneously building 7 means they would have to turn out around 70-80 combat planes a year just for their carriers. This is probably greater than their entire current annual entire combat aircraft production capacity.