In English, there are around 44 phonemes (sounds), but there are around 250 graphemes (letters or letter groups that correspond to a single sound). This is because every phoneme (sound) corresponds to more than one grapheme (letter or letter groups) across different words.
For example, the phoneme /f/ has four graphemes: f (as in funny), ff (as in puff), ph (as in photo), and gh (as in laugh).
The phoneme /ī/ has five graphemes: i (as in kind), igh (as in night), i-e (as in bite), y (as in try) and ie (as in pie).
Exactly because there is no one-to-one correspondence between phonemes and graphemes, children need systematic phonics instruction in order to learn the most common phonemes for each grapheme and the most common graphemes for each phoneme. This will enable them to decode (read) and encode (spell).
Below is a chart with the most common graphemes for each phoneme that children are taught at school.
Research