What percentage of men with bilateral undescended testes end up with normal fertility?

Less than 30% Percentage depends upon original position of undescended testes when corrected, i.e. Intra-abdominal udts are associated with more infertility than low inguinal udts. Associated abnormality such as poor attachment of epididymis has a worse prognosis. Age at correction is very significant, the younger the better. Both testes undescended beyond puberty can never be associated with normal fertility.
Few. If cryptorchidism (undescended testis) is not corrected before puberty then low sperm counts are common. The surgery is called orchiopexy and is performed by urologists.