The harmonics of a sound are the frequencies that are related to the fundamental frequency of the sound. The fundamental frequency is the lowest frequency that is present in the sound. The harmonics are multiples of the fundamental frequency.
A tuning fork produces a single sound wave, which has a single frequency. This means that a tuning fork only has one harmonic.
A guitar, on the other hand, produces a complex sound wave, which contains many different frequencies. This means that a guitar has many harmonics.
The number of harmonics that a guitar produces depends on the construction of the guitar and the way that it is played. A guitar with a longer string length will produce more harmonics than a guitar with a shorter string length. A guitar that is played with a harder attack will also produce more harmonics than a guitar that is played with a softer attack.