Most modern acoustic pianos have 88 keys—52 white and 36 black—covering a little more than seven octaves from A0 to C8. That range became a practical standard as piano makers expanded earlier keyboards to meet changing music, while notes beyond the extremes offered diminishing musical usefulness for typical instruments and repertoire.
Eighty-eight keys feels inevitable only because it is now so familiar. Earlier keyboard instruments and early pianos often had substantially smaller ranges, and the modern span emerged gradually.
The keyboard kept expanding
As piano construction improved through the 18th and 19th centuries, makers built stronger frames, more powerful strings and larger instruments. Composers and performers took advantage of the extra range, which encouraged manufacturers to extend keyboards farther into the bass and treble. The instrument evolved alongside the music written for it.
A modern 88-key piano normally runs from A0 to C8. That is wide enough to cover very low bass notes, the central registers used by much piano music and an extremely high treble. It also gives composers room to spread chords across multiple octaves and to create contrasts between registers.
Why not keep adding keys forever?
Extra notes are not free. Very low strings need length, mass and careful scaling to produce a useful pitch, while extremely high notes become short, bright and difficult to sustain strongly. Extending the keyboard also makes the case, action and stringing system larger and more complex. At some point, the musical benefit of another note becomes small compared with the engineering cost.
That helped 88 keys become a durable compromise rather than a physical law. It provides a broad range that supports the core classical repertoire and most later piano music without making every instrument unnecessarily enormous.
There are exceptions
Not every piano has 88 keys. Historical instruments may have fewer, compact keyboards often use reduced ranges, and a small number of specialty concert pianos extend beyond the standard bass or treble. Digital keyboards also come in many sizes.
So the answer is not that music requires exactly 88 keys. The number became standard because piano design, repertoire and manufacturing converged on a range that proved broadly useful. Once performers, teachers, composers and builders could rely on it, the standard reinforced itself.