You need to understand all of them. However, I only knew 4 poems really well, and the other two not so well. Basically, just try and remember some easy quotes for the ones you aren't planning to write about.
I honestly doubt you'll have that kind of question this year. You can analyse 3 poems in depth, in fact I encourage that, but just memorise some really easy quotes for the ones you aren't planning to write about. With Owen, he writes just about the same stuff for all his poems, so if you do get asked to write about a specific poem that you only know easy quotes on, you can substitute the analysis from your prepared paragraphs to fit the poem you aren't well prepared for. Also, don't neglect the title/structure in your analysis if you lack in-depth knowledge of the poem itself.