In the UK at least I don't think you *have* to publish in order to get your PhD, not in any field I know about anyway. Obviously it's good to do so if you can, and probably essential if you want to continue in research afterwards - how many papers to aim for depends a lot on the particular field, so it would be worth asking your supervisor, or look at what post-docs in your area did during their PhDs. This will also give you an idea of which journals would be appropriate for you to submit to.