As we already know from the previous answers that the difference between the sequence of requests will be like 1,3,7,15,31,63,127,511,1023,2047 and head can start from any position then we can start the zigzag line from backward i.e from 2048 to 1 (or we can do from 1 to 2048)
We can do it till the difference is 1.
So answer will be 11.
*As it is asked about the about cardinality of requeste set i.e the number of requests (not how many times it changed its direction) then it will be 11, as requests are 684, 681, 688, 673, 704, 641, 768, 513, 1024, 1, 2048.
*Here the head (683) is not included because head it self is not a request of the current request set.