As a digital printer (I run Gamer Printshop), printed maps are generally 300 dpi/ppi, however 200 dpi can print well too, the problem only exists for small font labels, the smaller the font the higher the resolution required to minimize pixelization. Most maps for publishers are usually 300 ppi but only to fit within a letter size printed book. However, those looking to print large format maps will require higher resolutions.

I design using vector software which allows me to create one map, usually designed in sizes from 18" x 24" up to my current map I'm working on which measures 48" x 72". From there I can export at full size 300 dpi, or any resolution down to VT resolution like 50 or 100 ppi. In raster applications like Photoshop or GIMP this is problematic in that the size and resolution have to be determined before you create the map. When dealing with many layers it gets memory intensive, so you might need have more RAM needs and more.