Answer to 1 - 2 flip flops will be sufficient. but it's not always the case. A ring counter has 4 flipflops still mod 4 counter. It's because some states are not involved in counting. A mod counter is a counter which can actually count something from it's counting states. a counter of 2 bit which has all states like . 0 goes to 0, 1 goes to 1 , 2 goes to 2, and 3 goes to 3. is useless and can't count anything.
Answer to 2 - special type of question and should look at it and understand it from the best answer there.
I think I answered all the questions.