You need to talk about one, maybe two (they'll usually ask for 'texts', which means more than one). But you should have between three and four up your sleeve- what if they ask a question, and you have no idea what to write with some of your related material? Better to be safe than sorry, although you can know some texts better than others to save time in studying all the texts (so have two-three you know well, and one-two that are emergency backups). Good luck!