or 11 of 'em in my new 125 galllon? haha--this rule of thumb probably only works pretty well for medium size tanks. (Small tanks aren't good for goldfish at all, of course.)
Here's a calculator that's based on surface area:
http://www.bristol-aquarists.org.uk/...o/aquacalc.htm
24 - 30 square inches for each inch of fish is another rule of thumb. The above calculator is based on 24 sq. inches and indicates five fish would even be a bit over the limit for my 125, if each fish grows to 12 inches length.