The test algorithm chooses the same number of words for each sound from each pair. For example, if you practice the sounds [b]/[p] and choose 20 questions, the test will have ten words with the sound [b] and ten words with [p].
However there are exceptions to this rule. Some sounds, such as [ʒ], are rare in English – there are only three words with this sound in the beginner's section. If you want to practice these types of sounds, you can choose more advanced levels.
By the way, all sound pairs inside each category are sorted by their frequency: the most common ones appear in the beginning and the least common ones appear at the end.
