True shubunkin fish are a type of slim type goldfish so would need a larger tank in the long run as they can grow to over 10inches long. However there are many fish with shubunkin type colouring (although technically these fish would be calico colour/pattern-wise). If you go for a fancy type of goldfish like a veiltail/fantail (which often come in shubunkin/calico type colouring), then you could have two in a 30gal long tank though, slim type goldfish (non-fancy goldfish) on the other hand need at least a 75gal tank minimum. Most fancy goldfish will grow between 5-8inches long depending on the variety.