Seven pre-production aircraft have already been built and the first two examples have reportedly been delivered to a flight test center at either Cangzhou, or more likely, Dingxin air base.
The first front-line regiment is supposed to activate and receive aircraft by June 2017 and the J-20 is expected to be combat ready by 2018 or 2019. That milestone could be pushed forward, given the budget priority. The final requirement could be between 500 to 700.
SOURCES- Combat Aircraft, Defense Update