There are 26 chapters total in Advance Wars: Days of Ruin. The names of these chapters vary depending on which version of the game you hold (see Regional Differences). The guides for each of the 26 chapters are all the same except for some parts of the storyline.
Each chapter contains an information box about the chapter, a storyline section letting you know what's going on with the plot, and official tactics from the in-game briefing room. The pages are finished off by a helpful guide for completing the chapter with minimal spoilers.
If you're ready, time to take command of your first battle in this new and devastated world: Days of Ruin.