The stocking levels depends on the following factors:
1. The temperature of the water.
2. The condition of the water.
3. Provision of aeration.
4. Type/grade of ranchu.

For item 1. The colder the temperature the more you can stock.

For Item 2. Freshly changed water allows you to keep more ranchu whereas water that has accumulated lots the ranchu's wastes will generally not allow high stocking levels.

For Item 3. The present of aeration serves 2 functions. First it helps to remove the CO2 from the water and secondly the agitated water surface and the stream of air bubbling from the air-stone allows air exchange to take place. This allows the depleted O2 to be replenished with fresh O2.

For Item 4. The smaller the ranchu, the more you can stock. For show grade or seed ranchu, generally the stocking levels are much lower.

For Inland Intensive Farming of Food fishes, the systems allows for ypp to 230kg of fish for every m3 of water. For more details you can refer to this site:http://www.aquaoptima.com.

Ranchu keeping and appreciation is a living hobby. Its "ALIVE". We should always look at technology to help us in our enjoyment of the hobby and falling back to the basics as our markers in developing this hobby. The shapes and profile of the Japanese ranchu has evolved greatly over the last 40 years mainly due to the advance in technology.

Experiment with various methods, meet up with fellow keepers, read magazine or books on ranchu or other types of fish including food fish farming and adapt it to your set-up. It is through interaction wit other keepers that you learn their success and pitfalls. What works for me may not work for you because of enviromental, physical, financial and time constrains.

I will summerised your query in this manner;

All things equal, will you prefer to keep,
20 - 3 inches ranchu or
15 - 4 inches ranchu or
10 - 5 inches ranchu or
5 - 6 inched ranchu or
2 - 8 inches ranchus

The choice is yours.