You *should* have a deep understanding of at least three of the readings, I think. The HSC will probably only ask you about one or two at the most (otherwise you won't be able to go into much detail AFAICS).
Don't choose readings which you think will get you 'good' marks, choose readings you understand and can write a lot about.
what our teacher told us to do is to know two scenes from about 4/5 different interpretations and to focus on these... ie. the opening scene, and the eye-gouging scene