Hi Deino, when you say first batch, do you mean first batch of j20a using ws10, if yes, what about those which use russian engines.Guys ... you need to understand the PLAAF serial number system. To make a guess there are already more than 75 based on a single number is like guessing the number of kids a pilot has based on the colour of its hair! It's impossible.
78233 means in fact 78x3x as standing for this particular unit - the 172nd Air Brigade at the PLAAF Cangzhou Flight Training Base and the xx = 23 means simply it is aircraft number 23. But this does not mean it is the 23rd J-20 within that unit nor the 23rd overall built. It is just a bort-number.
Otherwise I estimate, that there are about 36 built within the first batch equipping the two FTTB units at Dingxin and Cangzhou and the 9th Air Brigade at Wuhu. How many from the second batch are built I don't know.