There are 14 base characters. Which means out of the box you need to do 21.4 challenges with each character. Which is great because up to challenge 20 are really basic combos.
If you unlock Sin before doing this you only need 20 with each character.
Personally I did 20 with each. All of them on my main (Axl) and then the rest with Ramlethal. She has by far the easiest challenges. I did up to 27 with her and they were still basic combos.