Normally there is no need to include page numbers on your presentation slides. When you present in front of an audience and you are using slides, it is not really relevant for your audience on which slide you are. You can keep a clean look of your slides without including page numbers.
There could be a good reason to do include page numbers on your slides. This is when (part of) your audience is not in the room with you, but they are looking at the slides on a different location where they have to navigate themselves through your slides. In that case it may be helpful to include slide numbers on your slides, so when you move to the next slide, you can refer to a slide number, so the members of your audience who are not with you can keep track and follow your presentation.
