Question: How much planning is required when it comes to structuring a book?Answer:
It all depends on the writer.
Some writers (pantsers) can sit down with just a simple idea in their heads and start writing.
Others (plotters) create a very lengthy outline and extensive notes before they start.
Most writers are somewhere in between.
The trouble with being a pure pantser and not planning is that by the time you get to Chapter 10, you may find that choices you made in Chapter 1 make it impossible for the story to continue the way you want it to. Inconsistencies start cropping up because you made decisions without thinking them through. Or you may find you've gone down a path that takes you away from the story you wanted to write and you can't see how to get back on track.
Writer's block is often the result of painting yourself into such a corner.
On the other hand, there are many aspiring writers who spend so much time planning their setting, their stories, and extensive backstories, that they never get their book written. (High fantasy writers are notorious for this.)
You have to find the balance that works for you. And you always have the option of doing some planning, then some writing, then going back and planning some more, etc.
The important thing is to figure out what will keep you moving forward at a good pace. You know you're either doing too much planning or not enough planning if your output slows to a crawl. (Of course, your output also depends on how many hours a week you can devote to your writing.)