Like nlin said, powerpoints should just have bullet points. That being said, around 4-6 per slide is what I usually see/do, although it might vary from slide to slide. Some bullet points may have subpoints that you may want to outline. Main idea is: for each slide pick an argument relating to your thesis, and outline it in bullet form, then talk about it in more detail when doing your presentation. Some arguments might have more to talk about out, but if you're running out of space (judge by how "crowded" the slide looks) just make a new slide with the same header and continue there.