Allow none, one, or infinite; anything in between lacks rationale.
Van der Poel's rule. Any fixed limit other than none or unlimited encodes an assumption that will be wrong, which is why arbitrary caps are the most common source of later rewrites.
In practice
- A database field allowing exactly two directors meets its first company with three, and the change touches every screen that reads it.
- A form that allows exactly three phone numbers will meet its first four-number case within a year, and the rewrite costs more than the generality would have.
- Further reading
- Wikipedia
- Subject
- Design & Systems
- Essay
- Rules of thumb for building things that work