Self-help titles frequently contain a number: a count of habits, laws, principles or stages. The convention persists because the number does specific work at the point of sale.
A number describes the shape of the contents
A reader deciding between books has seconds and very little information. A counted framework tells them immediately that the book contains a defined set of things rather than an open-ended discussion.
It also implies completeness, suggesting the author has surveyed the territory and arrived at a settled list rather than offering scattered observations.
That impression is what converts browsing into buying, because the reader can picture what finishing the book would give them.
Finite tasks are easier to commit to
Behaviour change is daunting when it has no boundary. A count converts an unbounded ambition into a sequence with a visible end.
Readers can locate themselves inside the sequence, which supplies a sense of progress that an undivided argument cannot offer.
The same instinct explains why the counted items are usually presented in an order, even when the order carries no real significance.
An ordered list also gives the book its chapter structure, which solves the author's organisational problem at the same time as the reader's.
The count survives being forgotten
Most readers will not remember every item, but they will remember that the framework exists and roughly how large it is.
That residue is enough to recommend the book to somebody else, and recommendation is the main way titles in this category travel.
A number is also easy to quote in a talk, a summary or a conversation, so it moves further than the reasoning that produced it.
The number is chosen, not discovered
Material rarely divides itself neatly, so items are merged, split or renamed until the list reaches a satisfying size.
Small odd numbers are favoured because they read as considered rather than arbitrary, and because a very long list undermines the promise of manageability.
This is the point at which the framework starts serving the packaging rather than the argument.
What is lost in the counting
Ideas that do not fit the chosen structure are cut, and ideas that fit poorly are reshaped until they do.
Readers receive a tidier account of the subject than the evidence supports, which is the recurring trade the category makes in exchange for being useful at all.
The tidiness is not incidental to the form. A framework that admitted how ragged the material really is would be harder to remember and correspondingly harder to apply.