Using multiple pots and cooking the pasta beforehand are helpful tips when preparing spaghetti for a large group. If possible, utilize two or more pots at a time to boil the desired amount of pasta to al dente or slightly under al dente. Then store the pasta and finish cooking it later, right before serving.
Coat the pasta with oil or butter immediately after cooking to ensure the noodles don't stick together, then refrigerate the pasta in small containers to prevent further cooking. Reheat the pasta immediately before serving by placing it in boiling water for one or two minutes.
To determine the amount of pasta needed for a large group, multiply the number of people by 2 ounces of dry pasta, remembering that a pasta entrée and a pasta side dish require different serving sizes per person. Use about 5 quarts of water per pound of uncooked pasta. When cooking to al dente, check the pasta up to two minutes before its cooking time finishes to ensure it doesn't overcook.
Do not add oil to the water; added oil makes the noodles slippery and prevents the sauce from sticking to them. Likewise, do not rinse cooked pasta unless it's for a cold pasta dish, as this strips the noodles of important starches.