Recommendations there are some basic rules of thumb for foundation wall thickness and these are outlined in many modern building codes.
Poured concrete retaining wall height.
They are generally economical up to 10 feet in height for cast concrete structures.
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.
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.
Concrete retaining wall footing size the depth to the bottom of the base slab should be kept at a minimum of two feet.