I bet it's this one from Wikipedia, where 90% of 50 children experiencing gender dysphoria eventually came to terms with their assigned gender:
"In 1994, Zucker said that parents set the goals at his clinic. "We recommend that one goal be to help the child feel more secure about his or her actual gender, another to deal with the child's emotional difficulties, and a third to help with problems in the family. It's helpful to have parents set limits on things like cross-dressing, which many parents have not done before coming to us."Zucker's follow-up of 50 treated children found that "about 10 percent are still very unhappy about their gender, still cross-dressing, and thinking about having sex reassignment surgery" as young adults.[20] Zucker has stated that "the therapist must rely on the 'clinical wisdom' that has accumulated and to utilize largely untested case formulation conceptual models to inform treatment approaches and decisions."
**Note, both the small sample size and the fact that gender dysphoria is not identical with transgenderness make the statistic not apply the way that the OP is reading it.
