The phrase rendered knowing what is enough, or knowing sufficiency, appears in three chapters of this book and it is the nearest thing the text has to a rule a private reader can follow.
Here it is paired with knowing when to stop, and both are given consequences: no disgrace, no danger, and the possibility of lasting a long time. Chapter forty-six calls not knowing what is enough the greatest calamity there is and then turns the phrase into a small tautology, that the sufficiency of knowing sufficiency is lasting sufficiency. Chapter thirty-three lists it among its inward achievements and calls it wealth.
The three uses are consistent and they define a term rather than repeating a slogan. Sufficiency is not a quantity. It is a piece of knowledge about a quantity, and the chapters treat the absence of that knowledge as the operative defect. A person who does not know when enough has arrived cannot be given enough, because no amount will register as the amount, and the pursuit continues by its own logic rather than in response to any need.
That is why chapter forty-six can call it a calamity greater than any external misfortune. External loss has a size. An appetite with no stopping condition does not.
The idea travelled well beyond this book and became a commonplace in later Chinese moral writing and in the household maxims of ordinary life, often detached from anything Daoist. Its survival in that form is a fair measure of how much of the text's influence has been practical rather than doctrinal.