How to make a Tall Retaining Wall
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.