In words such as clock and kick, the grapheme ck is associated with the sound or phoneme /k/ - so there are two letters making one sound.
Notice that cat, kite, and duck all contain the same /k/ sound, but in each of these cases this sound is represented by different graphemes: c, k, and ck. They make the same sound but have a different spelling.
The reverse is also possible in that the same letter or combination of letters (graphemes) corresponds to different sounds (phonemes) in different words; the digraph ow corresponds to two different sounds in 'snow' and 'cow'. In general, in English there is no simple one-to-one correspondence between letters or letter combinations and sounds. Often the same letters are pronounced differently in different words. But these complexities don’t mean that words are spelled and pronounced in an arbitrary way. This is why it is critical to teach phonics to children in a systematic way in order to ensure success.
Overall, even though the same sound can be represented by different letters or combinations of letters, not all possible representations of a sound are equally common. For example, the sound /f/ can be written as f (funny), ff (bluff), ph (phone), or gh (laugh). But ph and gh are less common than f and ff. Systematic Phonics introduces children to letter-sound correspondences, starting with the most common ones and ranging through to the least common.
For example, the sound /k/ can be spelled c, k and ck. The sound /f/ can be spelled f, ff, ph, and gh. The sound /n/ can be spelled n, kn, and gn.