zettelkasten.de/posts/overview/ mentions one page to rule them all:
How many Zettelkästen should I have? The answer is, most likely, only one for the duration of your life. But there are exceptions to this rule.
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Zettelkasten is a note-taking and knowledge management system developed by the German sociologist Niklas Luhmann in the 20th century. The name "Zettelkasten" translates to "slip box" in English, which reflects its original form: a physical box filled with index cards (or slips, known as "Zettel") containing short notes and ideas.