Concerning resolution/sharpness for modern 35mm FF and APS-C systems the resolving power of the lens is more of a factor than megapixels on the sensor. For example, an 18 megapixel APS-C rarely produce sharper images than a 12 megapixel FF, since the lens is limiting. This makes it a bit risky to just consider pixels per inch when discussion resolution of prints. A 240 ppi print from a sharp fullframe lens can be sharper than a 300 ppi print from an APS-C with less good lens (assuming excellent photographic technique and conditions for both).
Wide angle lenses for DSLRs are often not so sharp, due to the required retrofocal design. A low cost 50mm lens can be much sharper than a (much) more expensive 24mm lens.
Stitching wide angle images using longer focal lengths is a *very* cost effective way to make high resolution images.
My experience concerning resolution of images is that you can indeed differ between different resolutions on prints, much easier than the "common recommendations", most recommendations in circulation is based on what limits old technology (film on smaller formats) had and what was acceptable, not so much the limits of a human eye in good condition.
However, an image that is not a sharp as it could be will still look good. Print technology of today is very good at hiding pixelation artifacts, so a low res print will not look blocky/aliased, just a bit fuzzy. This means that it is very much a matter of taste how much resolution you need. If you want to make prints that are so high res that further increase of resolution cannot be noticed even at quite close inspection, then you'll typically need 40 - 80 sharp megapixels. If you just need good looking results and don't worry much that it is not as sharp as it could be, 12 megapixels or so will usually suffice.
Above say A4/letter print size is not so much a factor, since smaller prints are inspected more closely than large, that is smaller prints will require higher resolution that larger. On the other hand, one might want to impress with resolution on the large print and have it sharp even when viewed closed only a part of the image, and then many megapixels will be required.