DPI is just a flag or piece of "meta data" in an image, it is not affected by (nor does it affect) the rendering process. If you have a specific print size and DPI in mind, simply multiply your intended DPI by your print size to obtain the needed image dimension in pixels, then use those for your render settings. Once the image is rendered and saved, you can adjust its DPI freely in any image editor. Basically, DPI really only matters when you print something.
- Oshyan