I only scored 6 times on the fastbreak and still got credit for completing the challenge.
10 points is indeed the requirement, not 10 fastbreak scores like the in-game pop-up text suggests.
That said, this game's awful fastbreak logic where all of your AI teammates just run into the paint instead of filling each of the appropriate lanes (middle, left wing, right wing, trailer, trailer) made it a lot more difficult than it should have been to score in transition.
The best transition strategy in this game is to just turbo dribble in a straight line with a cheese card that has gold posterizer and a 100 dunk tendency then just wait for the CPU defender to run back and stand in the paint for an easy contact dunk.