Question: How many sounds do you hear in the word box?
Answer: [a] [k] [s]. There are four sounds in all.
In English the sounds [b] [a] [k] [s] are distinctive (they have different sounds) and they are contrastive, like this,
Change the sound and a new word results. That means, [a] [k] [s] are phonemes in English.
Now let's try another word, say, sing.
Question: How many sounds?
Answer: [s] [I] [ng]. There are three sounds. Note, [ng] represents one sound.
Change the sound and a new word results.
Now try jumped. How many sounds do you hear? (Hint: try not to look at the word. Rely on your ears.)