When looking at modern car radios, it's obvious that the vast majority of these has 6 buttons for pre-set radio stations.
Why not less, why not more?
It was very tempting to add the tag radio-buttons to this question.
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The answer is to do with 'Subitizing' - identifying quantities at a glance rather than counting.
Most people can easily identify six items if ordered (e.g. in a row as the buttons are - or as two rows on a dice face).
Seven buttons would make it too tricky to locate a station.
Obviously 1-5 are also fine, but fewer buttons mean less stations.