I'm gonna chat with my dad. He's been doing Digi/photo and print for 20 yrs..
So spoke to him. See response below:
Regarding a 12mp picture, it depends on how you are printing it, on what type of machine you are printing—digital press, offset, large format. With large format, you can go down as low as 100dpi depending on what you are using the print for. Standard resolution for printing is 300dpi. If you are printing something like a picture of mountains, or people and you want to print it large, you can keep it at 300dpi, but your image size—width and height—will be smaller. With Photoshop, you can increase the size of an image, to a degree, without losing too much quality. I do it every day. So size is relative to how you print. I'm thinking a 12mp image might go up to 8x10 or so.