Well, since they are meant to be fed to big fish, I don't think that your
lfs will hold them in good conditions.
They will become big fish if you take good care of them. So yes, still 10
gal per fish, maybe you might get away with 4 goldfish in that tank.
The thing is that goldfish are messy eaters. And they poop allot. So when you keep too many, your water quality will never be able to stay within healthy limits for your fish.
Maybe if you add extra filtration, but even then you should also consider swimming space for your fish. You can cram allot of fish in a tank with allot of filtration, but that won't make them happy because they will be too crowded.
JMO