In the past guitar strings were made of prepared animal intestines, nowadays nylon and steel strings are most commonly used.
Nylon strings are normally used in classical guitars. Steel strings are used in other types of guitars. Strings made of other materials or combinations thereof are also occasionally used, e.g. steel-silk strings.
There are twelve-string guitars in which each string is doubled. The top two strings are tuned in unison, and the remaining strings in an octave interval.
Seven-string guitars also appear in Russian and Mexican folk music. They are also used by some modern metal bands to obtain "heavy" sounds by using lower registers.
An eight-string guitar is also found - in Poland it was patented by the Krakow guitarist Krzysztof Sadłowski for the needs of the Warsaw Autumn festival. At this festival he performed the piece by Anna Zawadzka-Gołosz Concerto for eight-string guitar and strings with piano.
Ten-string guitars are also being constructed. The difference between such a guitar and a traditional 6-string guitar is the addition of 4 bass strings. The introduction of additional strings has, among others to faithfully perform Renaissance and Baroque guitar music. Many lute songs of that time require a larger scale than a regular guitar would allow. Performing such songs on a 6-string guitar is only possible by transcription. The propagator of such an instrument was the Spanish classical guitarist Narciso Yepes. Wikipedia