When my only DSLR was a 6mp D50 I made several 20"x30" prints, and they look great. I made the first one just as an experiment to see how it would come out, and was so happy with it that I made more. Now, with my 12mp D300 I have more than enough mp. While the largest prints I have made are 20"x30" (except, see below), I rarely print that big, so for the most part I feel 12mp is more than enough (that's what I answered) and that 6mp is even enough. I still use the D50 because for many purposes, it will return results as good as my D300 (when the light is good, I know I won't print big, for example).
All of my 20"x30" prints were uncropped, so I can see that if I was cropping, I might want more mp. But it is rare for me to print that large anyway.
My largest print, from my 12mp D300, was 4 feet by 6 feet! I took a team photo of a high school basketball team and gave a jpg to the coach, and he had it blown up and put on the wall of the gym. I don't know how it was made, but I was amazed at how sharp and clear the photo was, even at that size, from 12mp. Despite occasional bouts of NAS for one reason or another, I never have mp envy, and am completely content with my 12mp.