When the race begins, there are eight racers. Four of them crash, plus Darcy takes out at least four or five, then she saves Riven. Not counting Darcy, that makes nine - ten racers in all, which is the wrong number.
Ignoring the fact that she's a late-comer, or the fact that the race is boys-only, Darcy should never have been allowed to stay in the race long enough to save Riven. Her telekinetic blasts she uses to knock out the other racers should have gotten her stopped after the first or second one.