A common rule is 20 gallons for the first goldfish, then 10 gallons for each additional fish. I disagree with this, since I don't think 30 gallons is enough room for two goldfish. I would not keep goldfish of any kind in anything less than a 4 foot tank, since even fancy goldfish grow to 8" when fully mature. They are long lived fish, so deserve the extra space to live in good health for their long lifespan. They also produce lots of ammonia for their size, and are very deep bodied...which is why they need more space than most other fish.
Anyway, assuming the bare minimum, you would need at least a 75 gallon tank for 6 fancy goldfish. I would say you need more like an 120 gallon tank for this many goldfish, and a 6 foot tank. That's 455 liters.
Is there any way you can trade some back in to your store? Best to do it while they are small.
For your sized tank, I think two goldfish is plenty in the long run...you can keep three or four while they are young, but as they grow they will need to be thinned out, otherwise you are going to have water quality trouble and possible stunting.