Can I convince you to not do this ?
Honestly not trying to be an ass.
Please, please - Get a quote from a company that builds retaining walls.
The quote will cost very little or nothing.
You will be shocked at the cost, and not in a good way.
The quantity of material needed to build the wall, will amaze you.
Please get a quote or two first.
I promise you, you will not regret it.
Block walls work in compression, heavy things directly on top of them is fine.
Side to side, their goal in life is to tip over, and they are damn good at it.
Blocks work because they are tied to very solid, heavy things at the top and bottom that run the length of the wall, and have regular supports at a right angle to the length of the brick.
A 1' deep pile of soil, behind a 4' block wall, 1 block long, weighs 500 pounds.
The path of least resistance is to knock over the wall.
Blocks, attached with mortar, to the soil behind the wall about as strong as a 2x10 resting on its edge.
The question is more one of how many yards of concrete will you need to build the footing.
Building 23' of retaining walls is not a small task. This is a giant undertaking. So be forewarned. :)
As stormy noted, 4' is typically the height you can go without having to trigger an engineering sign-off. However, some areas may also have a total height requirement as well. Personally, if I was buying a house with 6 levels of 4' retaining walls, I'd want to see an engineers signature before making an offer. That's a lot of land that's being held back.
As for how far you need to step back, the minimum is the height of the wall. So a 4' high wall would need 4' of space between it and the next 4' wall. More would be better.
Best Answer
A good starting point would be to plan on burying one course on a minimum of four inches of compacted gravel. The buried blocks don't buy you much stability in that configuration but they will help keep the wall from sliding forward and prevent the ground washing out underneath the wall, which is unsightly.
The gravel base must provide enough drainage to prevent your wall from becoming a bathtub. Barring truly epic monsoons or snow melts a modest depth should suffice. The base, by the way, should be twice the thickness of the wall. I doubt that's an issue since a ditch narrower than 8" would be a real hassle to shovel or tamp down stone in.
That basic plan should be modified by the local frost line, soil composition, and common sense. If an extra couple inches will get you below the frost line that's good insurance against heaving. If going below the frost line means digging an extra two feet that's a good reason to stop early and have a beer. Remember, you never get perfect compaction even with a machine. An exceptionally deep base that's 90% compacted will still settle a couple of inches.