The short answer
It comes down to tank size and goldfish type, not a fish-per-litre rule of thumb. For fancy goldfish, budget about 75 L for the first fish and ~40 L for each additional one. Single-tails (commons, comets, shubunkins) grow large and fast, so most tanks only suit one — they’re really pond fish. Understock rather than overstock: goldfish are heavy waste producers, and crowding is the fastest route to poor water and sick fish.
Why the numbers are generous
Goldfish combine large adult size with a huge bioload. Pack too many into a tank and ammonia and nitrate climb between water changes, oxygen drops, and disease spreads easily. Giving each fish plenty of water keeps chemistry stable and lets your filter keep up. It’s always better to keep fewer goldfish well than more goldfish badly.
Do goldfish need company?
Goldfish are sociable and often more active in a group, but they are not schooling fish and a single goldfish is perfectly happy alone. If you want more than one, add the space to match — don’t squeeze extra fish into a tank that’s already full. And keep goldfish only with other goldfish: they need cooler water than tropicals and will eat fish small enough to swallow. See can goldfish live with tropical fish?
Plan before you buy
Work out your final tank first, then decide numbers. A common goldfish or comet needs a pond or a very large tank; fancies fit a big aquarium. Browse the aquariums hub and best large aquarium guide, and read how big a goldfish tank needs to be to size it right.