In general the top of the stem of any cast concrete retaining wall should not be less than 12 inches for the proper placement of concrete.
Poured concrete retaining wall height.
For example if you are pouring a retaining wall that will be 12 inches thick and will use wooden planks that are 1 1 2 inches thick for your forms you ll need to space the stakes about 15 inches apart 12 inches for the retaining wall plus two pieces of 1 1 2 inch thick wooden board.
Recommendations there are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes.
They are generally economical up to 10 feet in height for cast concrete structures.
Taller walls backfilled with heavier soil risk failure unless correct design methods are followed.
Backfilling with a clay soil instead of sand can increase the load by double or more while going from an 8 foot to a 10 foot wall just a 25 height increase adds 50 to the lateral pressure.