In calculating the retention index, why do we use the character state with the lowest frequency?
The maximum number of steps (or changes) is the number of taxa with state 1 or 0, whichever is smaller. Conceptually, this is the number of character changes if each taxon evolved its state independently of the other taxa. 

If for a given character the character state 0 occurred twice and character state 1 occurred five times, 2 would be the maximum number of changes.