I'm not an expert (and I'm sure one of the appeal panel members will correct me if I'm wrong) but
Having looked at the Q&As and other topics I believe what I need to know for the hearing of prejudice is the number of places at Shottery and the actual numbers of girls they take. Is this correct?
Yes. The case from the school should tell you why there would be prejudice. If the panel decided that there wouldn't be prejudice to the school (which I imagine is highly unlikely) then they would allow the appeals without needing to move to the second stage.
If the panel decides there would be prejudice, they move to the second stage, which is the balance of prejudice, ie would the child or the school suffer greater prejudice if child was / was not admitted. This is where you get to put your individual case.