Definitely do more than one.
Choose two which link together, but maybe one of which contrasts your other texts in some way.
Using more than one poem is advantageous for showing an understanding of a wider range of values, the only problem is really knowing how much depth you need on each one. It depends how you structure you essays, but you might be able to talk about them at the same time by focusing on their common values, using examples from each, if that make sense.