There are three phonemes in the word "hymn". Hard to show what they are without a phonetic font, but they are the "h" at the beginning, the vowel in the middle, the same as the one usually written "i", and the "m" at the end.
A phoneme is the smallest sound unit of a word when you pronounce things, such as the "s" sound or the "th" sound for example. Phonemes are often, but not always, represented by letters. The word...