Eight megapixels is sufficient for an uncropped 8x10 of virtually any subject. 'Nuff said on that.
But enlarging more requires more resolution for most subjects. Basically, the original image must resolve all the detail a viewer will expect to see of a particular subject at a particular enlargement.
For portraits, as an example, any time a face is enlarged to two inches or more, the facial hair must be resolved. If viewers can't begin to count eyelashes on the eyes of a two-inch head, they will not consider the image sharp. Hair is easy to interpolate--it's just a line--so if the facial hair is resolved on the original image, you can interpolate it successfully to nearly any size.
That's why headshots are successful even at 4mp...at that scale, the facial hair is resolved even at such a low resolution. But if you expect to shoot full-length group portraits and successfully enlarge them to 2-inch head sizes--"successfully" meaning resolving the eyelashes--you need at least 20mp.