1 Some people in the school will get 20, others will get 12. It depends on your household registration, and ordinary residents will be assigned 20.
But you said that your hukou is south of Shenchuan Avenue and belongs to No.4 Middle School. If you study in no.20 middle school, you have to pay.
Personally, I think 20 is better and 4 is just as good. If you are in junior high school, 20 is similar to 4, so I suggest you don't have to spend extra money to read 4.