A full-sized piano has 88 keys; 52 white and 36 black that span seven octaves. Originally, pianos were modified harpsichords and had 60 keys that mimicked its predecessor's five octaves.
During the Renaissance, Baroque and Classical periods, composers used the five octaves exclusively; any piece of music from this time is played solely of 60 keys.
In the mid-1800s, pianos began to be made with a full set of 88 keys by the end of the century, Steinway Piano Company came out with a standard model with a full seven octaves. Soon afterword, other piano manufacturers adopted the 88-key model as an industry standard.