The earthen trout pond is a reasonable, manageable operation. The critical factors are water supply and waste. The first, and its exchange flow rate will determine the fish capacity of your pond. This is always calculated on the basis of full-grown fish, not fingerlings. Your local county may have regulations around the use of public water and disposal of waste water which may need to be satisfied.
Many hobby farmers are able to tap their own wells or springs for sufficient water.
In some places, where supply is an issue, recycling systems can be obtained which will support up to ten re-uses before replacement. Pond capacity can be calculated in two ways. Capacity, then, is a function of the ratio surface area, inflow rate, and oxygen demand of sediments. The second method, the density index, is perhaps easier. Without factoring water flow, permit 4.5 pounds of mature fish per cubic foot of water.