People speak easily about serial production of aircraft, today for example wrt J-15, but it more practical to produce Flankers in batches, as happens with J-10. As Totoro explained it in #3642 of the current J-10 thread:
This is even more relevant for Flankers as there are four variants in production or soon to be in production - J-11B, J-11BS, J-15 and J-16. They can be produced in the same plant in succeeding batches of what size? 20?it is more likely it denotes fourth batch, eight plane. 6th batch had 40 planes, earlier ones should have less.