Pre-measure how much spaghetti you need for how many people you will need to serve.
One full box (which is around 454 g [~1 lb]) of dry spaghetti will feed approximately 4 to 5 people. One serving is equal to only 1/4 lb (100 g).
For every 100 g (1/4 lb) of spaghetti to cook, add 1 quart (4 cups) of water. If you're cooking a full boxful, you will need 4 quarts of water to cook all that spaghetti.