Just to clarify, I use the term topics differently than most. No matter how someone views a topic, the advice of a set number of keywords is completely wrong when you consider how search engines view topics.
I will explain how I see this point and why I use the term topic the way I do.
When I refer to topics, I am referring to semantic topical analysis. Each word fits into one or more topics and with disambiguation just one topical area. It is a hierarchy where topics of a work is scored to determine what a work is about. A work will fit into more than one topic area, however, be strongest in one or few topics in particular.
This image is taken from Tropes.
When I refer to topical strength, I am referring to the scoring behind this analysis. The reason why this is important in search is because keywords are misleading. I often tell a story about a guy who called me frantic that he was receiving searches for women's clothing when he was clearly writing about cars. His problem was that he was using colorful language to describe the cars such as sexy, curves, and others such as leather, vinyl, hugging, etc. This was in the early days where semantics was being used more albeit inartfully in this case. The decision was to wait for Google to catch up with its mistake. It did not take long to Googles credit.
I mention this because words have more than one meaning. Focusing on a handful of keywords misses the point and takes away from the topical scoring potential. Full context is what is important. Here is another example.
This shows more analysis of the work. Notice factive, comparison, time, place, and manner. All of this is important to understanding content. In fact, this is a piece I wrote about an historical place both when built and today. I use facts as the basis of the work, but also colorful descriptions to engage the reader. So what does the analysis see? Just what I wrote about. Not keywords. But the content itself to be fully understood. Why is this important? You cannot match search intent with keywords. But you can with semantics. Anyone searching for facts on the location would find it in my content. And with strong topical scoring, it is likely to be found before weaker scoring content.
Remember the days where SEOs thought Google was grading grammar and spelling? In a round-about way they were. Semantics requires clear content to be analyzed. The topic must be clear, the emphasis must be clear, the point must be clear. Google was not grading content, but rather poorly written content was not scoring as well as well written content.
How you apply this to your site is up to you. However, I strongly advise when you write about a topic, you be as thorough as you can be and limit your scope as not to dilute it's search potential. In otherwords, write like a writer. You can use several pieces of content to complement each other and make your point. Organize them according to how they relate to each other and link between complimentary topics. Organization when it comes to content is less a computer science thing, and more a writer thing. For me, SEO is more about the disciplines of writing before getting down to the other off-page, on-page topics. It really makes a difference.
As for your example, I simply see this as topic/subtopic much in the way a file would be placed into directories. The reason why this may be limited to two or a few is because generally this works best for search though not exclusively and uniformly accepted in the blog world right or wrong. This is freeform. Many times people will change the organization of the site as they go. This is a normal discovery process. Whatever you end up with should make sense semantically. The categories and topics should score within the topic hierarchies found within the analysis. Otherwise they may be ambiguous.