how many 071 and 075 does China need for an invasion of Taiwan?
This is a wide open question. The answer depends on how the compaign will start and how it will end.
If PLAF and PLRF alone will be able to force the secessionists to surrender, then none of the 071 and 075 will be needed. On the other hand, if PLGF foot soldiers will have to take Taibei or Gaoxiong street by street, then all the landing ships, and more, will be required.