Block count for a segmental retaining wall is wall face area divided by block face area. The part that decides whether the wall is still straight in ten years is underneath it.
The base is most of the work
Dig a trench about twice the block depth wide and deep enough to bury the entire first course plus six inches. Fill with six inches of crushed stone with fines and compact it in two lifts.
Level the first course obsessively, block by block, in both directions. Every course above copies whatever the first one does, and an eighth of an inch of error at the bottom becomes an inch by the top.
Bury the bottom course
One inch of buried wall for every foot of exposed height is the standard. A three-foot wall has its first course three inches below finished grade.
This is not optional trim work — it is what keeps the base of the wall from kicking out under load.
Drainage decides the lifespan
Water behind a wall is the force that pushes it over. Backfill the first foot behind the blocks with clean crushed stone, not soil, and run a perforated drain pipe at the base out to daylight.
Soil backfilled tight against the block traps water, freezes, and levers the courses apart.
Know the height limit
Most manufacturers cap unreinforced block walls at three to four feet. Taller than that needs geogrid reinforcement layered into the backfill, and many jurisdictions require an engineered design and a permit above four feet.
Two shorter terraced walls are often easier and cheaper than one tall one, and they look better.
Backfill is a material you have to order
The gravel behind a retaining wall is not optional and it is not a small quantity. A typical specification is a foot of clean crushed stone the full height of the wall, from the base up to near the top.
That is a real volume — on a twenty foot wall three feet high, roughly two and a half cubic yards of stone before any base material.
Its job is to stop water building up behind the wall. Water pressure behind a retaining wall is what pushes them over, and soil backfilled straight against the block holds that water against it.
The drain at the bottom
The gravel moves water down; a perforated drain pipe at the base of it moves the water away. Without the pipe the gravel fills with water at the bottom and the wall is back where it started.
The pipe sits at the base of the gravel, wrapped in filter fabric or sleeved, sloped to daylight at one end where the water can escape.
Filter fabric between the gravel and the retained soil is the other half of it. Without that separation, soil migrates into the gravel and blocks it, usually within a few years.