I want to give a short, conclusive answer: Ten.
My photography instructor, a man who has done panoramic shots of the Arctic region, covered sports events and done billboards, said so. He did a giant billboard at 10 megapixels.
Megapixels, for the uninitiated, is not a measure of photographic quality, but rather of actual picture size. So while a cellphone with an 8 megapixel camera may sound impressive, it simply means you can make large prints of a photograph with cellphone quality. Quality is determined by your lens and the size of your camera sensor. The bigger the sensor the better the camera will process your images. For these reasons a compact camera or cellphone will never match a big DSLR.
So there's your answer: 10 megapixels is all you need.