fancy goldfish being the twin tailed egg bodied fish, not the slim common goldfish,i know these are too big. im just wondering how many of these you would keep permanently housed in a 50 gallon (4foot long) tank.
You could keep about 4. Gold fish are messy fish and need a good filter and 25% water changes weekly. Make sure the gold fish are fancy, not commons, Shubunkin, or comets. If they are any of those three you would only be able to keep one of them in your tank.
Truthfully, I would only keep one in a 50 gallon to be on the absolute safe side. And even then, I wouldn't allow it to be permanently because they really aren't meant for anything other than a pond/lake.