It is clearer if you think of it as partial pictures of 24 boys, 12 on the inside and 12 on the outside. In either position, each picture on the inside lines up

It is clearer if you think of it as partial pictures of 24 boys, 12 on the inside and 12 on the outside. In either position, each picture on the inside lines up with one on the outside, and vice-versa (each position is a bijection.) In position B, each of the pairs form a picture of one boy, leading us to count 12. The same is true for position A, except for the one case where each part-picture is well more than 50% (and, in particular, there are two complete heads.) Consequently, this cannot be seen as a picture of one boy, and we count it as two. In this view, the answer to your question is 'one of each pair, except in one case in position A only.'