Ali Cobby Eckermann's "Inside my Mother" is a prescribed text for Module A, not Module B.
I do not think that memorising entire poems is a good approach for your trial exam and HSC exam. Not only would you be overloading your memory with parts of a poem that are not as useful, but memorising multiple poems overlooks important aspects of effective preparation. Instead, your preparation should involve studying notes targeting a number of poems (including information such as techniques and quotes used in each), as this equips you with a good understanding of each poem while avoiding the unnecessary barrier of memory overload. Depending on the way in which you have approached English Standard throughout year 11 and year 12, you may wish to consider writing an analysis component targeting a number of poems, maximising its quality by seeking and implementing your teacher's feedback and then memorising it in preparation for your trial or (if there is not enough time to do this before your trial exam) your HSC exam.
To answer your question, when I did my HSC, I studied Kenneth Slessor's poetry for the Common Module. I chose to analyse two of his poems in depth and this allowed me to achieve 17/20 for that essay. Based on this, if you are able to present a sophisticated analysis, exploring two poems should be sufficient. Otherwise, you may wish to consider exploring a third one to consolidate your analysis.
I hope this helps!