You need to use a large volume of water in a large enough pot—that is to say, enough water for your pasta to be submerged the entire time it's cooking and enough room in the pot so the water doesn't boil over. If you don't have enough water or room in the pot, you could end up with undercooked pasta, and that would be depressing. As a rule of thumb, use 4-6 quarts of water for every pound of pasta.