For the best results:
As many as you can afford.
But consider that you also need light for all those pixels - meaning many a 5 MP camera actually is no better than a 4 MP camera due to too small a lens.
If you want to print at 300 dpi, you're bound to get decent results if you use the following formular
Size of Image = Pixels / 300.
In reality, if people keep within the 'optimal viewing distance' you need no more than 1.5 MP for *any image* to look sharp.
Alas, as soon as you tell people 'I made this with my digital camera' they tend to bump the picture into their nose.
And if you want to enlarge parts of images, you obviously need more pixels.
