In all mammals, the testes descend from the abdominal cavity into the scrotum after reaching a certain stage of development. This process has a specific term: testicular descent. In mammals with a scrotum, testicular descent, which maintains a testicular temperature lower than normal body temperature, is essential for sperm formation. 
In cats, after reaching sexual maturity, the testicles normally descend from the abdominal cavity into the scrotum. However, if one or both testicles fail to descend into the scrotum, this condition is clinically known as cryptorchidism.
It is possible for cats to be born with only one testicle. However, without proper examination, it is not possible to distinguish from external appearance alone whether it is a case of only developing one testicle or cryptorchidism.
Furthermore, the condition of a cat being born with only one testicle is extremely rare in clinical practice; more often, cats have cryptorchidism (undescended testicles). Therefore, a veterinary examination is necessary for a definitive diagnosis.
