Before you consider building your the racks for containing your fish tanks, you should think about how many tanks you will likely install in the long term and their sizes.
Since you will need to supply air, light, and heat to the tanks, a limiting factor will be the convenience of electrical outlets near your tanks, and the amount of electricity already drawn from the outlet by other devices.
For reasons outlined previously, the guppy breeder usually has many small tanks and a few large tanks. These are usually arranged lengthwise, side-by-side to preserve space and make them easier to maintain.
In general, smaller tanks (5.5 gallons or less) are used as maternity tanks and raising fry. The small size allows the fry to find food easier, expending less energy, and promoting growth. Larger tanks are used for juvenile fish. Some breeders use big tanks (20 gallons or larger) for growing out males or virgin females. However, tanks larger than 10 gallons are not really necessary, since fish from a drop can be divided among tanks of this size. Some breeders do not use tanks over 10 gallons. The author likes tanks 15 gallons in size.
Japanese breeders, who on average must find space in small living quarters, set up small fish rooms with 5.5 and 10 gallon tanks.