Normally, males are born with two testes. The function of the testes is to produce sperm.
However, in certain cases, a child only has one testicle, or the testicle is not descended from birth. This condition is also known as monorchism.
The following Thevolume.net summarizes information about one testicular abnormality or monorchism reported from Healthline:

What is monorchism?
Monorchism is a medical condition in which a man only has one testicle. This condition usually occurs during the embryonic development period.
Even so, this condition does not have any symptoms when the baby is still in the womb but is only known when it is born.
Chyptorchidism, the cause of one testicle not descending into the scrotum
Various factors cause monorchism. These include cryptorchidism, a condition in which only one testicle descends into the scrotum, while the other does not. Cryptorchidism is mostly experienced by baby boys who were born prematurely.
During fetal development, around the age of 28-40 weeks, the testes normally enter a canal called the inguinal, which is the tube that forms the way for the testes to descend from the abdominal cavity to the scrotal sac. However, in the condition of cryptorchidism, these testes do not go to the scrotum.
Vanishing testicles, cause one testicle to disappear
Testicular vanishing or testicular regression syndrome is another condition that causes one testicle to disappear during development. In boys who experience this, the disorder tends to go undetected.
Vanishing testicles can be caused by testicular torsion, injury, or a hormonal imbalance during pregnancy. The body’s immune system picks up the signal that the testes have been damaged so that the white blood cells become active and think the organ is not functioning so they are removed.
Orchiectomy, the testicular removal procedure
In addition to the two congenital factors above, the condition of only one testicle that is owned by a man can be due to the procedure of removing one or both testicles at once. This surgery is performed for various reasons, such as testicular tumors, serious injuries, prostate cancer, and others.
Treatment and treatment of one testicular abnormality in boys
If the congenital condition of one testicle is identified early, it should descend spontaneously in the first four months of birth. But if it doesn’t go down, the doctor will suggest a surgical procedure called orchidopexy to lower the testicles into the scrotum.
As for the problem of testicular vanishing, this condition cannot be treated. Further examination is needed to ensure that the conditions that occur are cryptorchidism.
Does having only one testicle affect fertility?
The thing that is feared can affect the lives of sufferers of one testicle is its relation to fertility. But don’t worry, many people with one testicle have a healthy and active sexual life like any normal person.
One testicle can produce enough testosterone to fuel sexual arousal, erection, and ejaculation. People who have only one testicle can also produce sperm for pregnancy, as long as they are healthy and do not have other fertility problems that affect their fertility.
I hope this information is useful, Mom.