They are generally economical up to 10 feet in height for cast concrete structures.
Poured concrete retaining wall height.
Poured concrete foundation walls that are less than 8 feet tall and have soil outside that is 6 or 7 feet deep against the wall can often be 8 inches thick and function quite well.
Recommendations there are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes.
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.
Taller walls backfilled with heavier soil risk failure unless correct design methods are followed.
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.