If you memo twists/flips with 2 letters then I think the maximum is 18 pairs(36 letters). If you memo twists/flips visually(so counting them as 0 letters), I'm not sure(based on what I worked out it would be 11 letters(22 pairs) but I'm pretty sure it's more than that
Whenever you finish a cycle and you think there are pieces left, just go through the story and cover every letter you encounter