Which instrument has 88 keys?

Eighty-eight keys, 52 white and 36 black, spanning a little over seven octaves. The number settled in the 1880s, when Steinway built instruments with that range and the rest of the industry followed. It is not a law of physics - Bosendorfer makes concert grands with extra bass keys, usually painted black so pianists do not lose their bearings.

The range was chosen because it is about the limit of usefulness. Below the bottom note the strings are so long and slack that the ear struggles to hear a pitch at all rather than a rumble; above the top note the strings are so short and stiff that the tone is more click than music. The piano covers essentially the whole span in which a note sounds like a note.

Under the lid is a lot of engineering. A concert grand has around 230 strings pulled to a combined tension of some twenty tonnes, held by a cast-iron frame that exists purely to stop the instrument folding itself in half. Bartolomeo Cristofori's original invention around 1700 was about the hammer action - a mechanism that throws the hammer at the string and lets it fall away instantly, so the note can be loud or soft depending on how hard you hit the key. That is what the name says: pianoforte, soft-loud.