Depends. The rule of thumb I go buy, in some cases give or take, is 55G for one, ONE Common Goldfish, and 30 for ONE Fancy Goldfish, like a Moor or an Oranda.
If you have multiple Goldies, you can chop of a few gallons. Like for my pond, each fancy will get 25 gallons, bit there will be a few empty gallons so I guess thats for them anyways.
And 75 for a Pleco.
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs