Here we are going to make a flip-flop based 4 bit counter. ie it counts 0000-1111 in a binary system. The clock of the first flip-flop is either natural crystal clock or output of 555 timer IC =. As we know if the J and K input pin of the flip-flop are both high then the output toggles every clock cycle. So all J and K terminal of ff is made high and all of the output toggles with some delay and hence there it can count up to nth flip-flop assuming each flipflop is one bit. If you wanna increase bits then you need to increase flip-flops.