A standard piano has 88 keys, but well over 200 strings. Typically the lowest octave or so consists of single-string notes, or monochords. The next section, usually ending somewhere in the octave below middle C, consists of two-string notes: bichords. All monochords and bichords are wound strings, meaning they have a steel core but have one or two layers of copper winding on top which increase the size of the string and yield a lower pitch. The rest of the piano consists of three plain-wire strings for each note.
And that is part of why it takes a while to tune a piano. Not to mention re-stringing one!