A meal to feed 30 guests would require approximately 4 pounds of spaghetti. A typical portion of pasta is 2 ounces dry; sixty ounces is equal to 3.75 lbs. In a large group, a few extra portions is a good idea.
A 2 oz portion of dry pasta weighs about 5.5 oz when cooked.
Cooked, 2 ounces of dry spaghetti has a volume of 7 to 8 ounces. Consider how the food is being served. A buffet or family style requires extra pasta because people tend to take a larger portion. If the meal is plated and served, be sure your portions have sufficient volume on the plate or are not too large for it.
If you don't have a large pot to cook a large batch of noodles, you can cook it in smaller batches, chill it, toss it with a small amount of olive oil, and reheat it when needed by dipping it in hot water for a moment.
Cook your pasta in heavily salted water. This ensures a consistent temperature throughout the pot and provides a little extra flavor
Be sure to stir your noodles 15 to 20 seconds after adding them to boiling water to prevent sticking. Stir occasionally throughout the cooking process. Typical dry spaghetti takes 10-12 minutes to reach "al dente."