Custards are a mixture of milk and eggs, and are the magical ingredients mixture not only for delicious ice cream, but for quiches, puddings, and other fabulous dishes as well. Whether you use equal amounts of egg yolks and whites, or one more than the other, the general ratio is three eggs to thirty-two ounces of dairy, which makes a custard that will set up well.
You need to temper the eggs (which means to beat them while pouring the hot milk or milk and cream mixture into them) in order for the eggs not to scramble instead of custardizing. (If you end up with scrambled eggs anyway, as happens to the best of us, don't worry, you can rescue the dish--keep reading!) Most people will tell you that the milk/egg mixture needs to be heated until it coats the back of a wooden spoon, but I find that heating it to 78º C means your custard will be firmer and set up better. The mixture begins to coat the back of a wooden spoon at about 72º C.
And now, what to do when you end up with scrambled eggs, despite your best efforts at tempering? Not to worry--haul out your trusty blender, set it to the highest setting, dump the whole lumpy mess in, and let your blender do the work for the next twenty seconds. (Or, you can just start the whole thing in the blender to begin with.) I dare anyone to tell the difference--in fact, the lumpy scrambled eggs floating around in the milk will be pulverized to an amazingly smooth creaminess. And there you have it--your dish is rescued, and no-one need to be the wiser. (You can also clean your blender and have it put away in about a minute, by putting in water and a drop or two of dishwashing liquid, filling the blender container halfway, and blending for another thirty seconds. Rinse, dry, and put away. Voilà, your secret is safe!)