How many keys does a standard piano have?

How to get close to this number

Build it from octaves. An octave has twelve keys — seven white and five black — and a piano covers a bit over seven octaves. Multiply and add the few extra keys at the bottom end, and you arrive at the answer. Reconstructing from the repeating unit is far more reliable than recalling the total.

The reason for the range is the interesting part and it is stated in the explanation: it covers essentially everything the human ear perceives as distinct musical pitch. Beyond the extremes, notes stop being heard as melody and start being heard as rumble or as a whistle, so extending the keyboard further would add keys that nobody could use musically.

That makes this a rare case where a standard is set by human physiology rather than by accident or convention. The marathon distance and the golf hole count on this site came from arbitrary historical decisions; this one came from the limits of hearing, which is why it settled in the late nineteenth century and has never needed revision.