Many people react very skeptically to the large tank volumes recommended for goldfish. The short answer is that goldfish are different from tropical fish. Their keepers also are different. When you obtain a goldfish as a pet, you are obtaining a responsive and intelligent (for a fish, anyway) creature that you can expect will live as long as a dog or cat would. Based on this, the goldfish keeper is a collector of individual fish rather than a collector of indistinguishable schools of interchangeable specimens.
One thing that creates a misconception from the start is that pet shops often have dozens of goldfish in a single tank of perhaps 30 or 40 gallons. Remember that (1) these shops have centralized filter systems -- the fish may be in the equivalent of a 2000 gallon tank -- and (2) pet shops often have high mortality rates, so you shouldn't assume that that's the best way to do things at home.
Never think that you need to buy a dozen fish because half will die in the first week. This is a self-fulfilling prophecy. With this in mind, take the time to select one or two goldfish that hold special attraction for you when you start out. Then give them the care they need to thrive. You will quickly find that a tank with a pair of special fish in it doesn't seem empty at all.