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.