How many flip-flop are needed to divide the input frequency by $64$?
Giving an output frequency of $2^n$ (n=number of flip-flop) requires n number of flip-flop.
so in order to divide the frequency we need n flip-flop . Here $2^n$ = 64 => $2^n$ = $2^6$
therefore n=6 is the required number of flip-flop.