Is it normal to have one testicle?
Men normally have two testicles. There can be a number of reasons why you have only one. The most common reason is that one of your testicles did not descend when you were born. This happens in about 3% of all births. Prior to birth, a boy's testicles are in his groin, but they move into the scrotum just before birth. Typically doctors check for this and surgery is done to move the testicle down into the scrotum. However, if that cannot be accomplished in time, the testicle dies and has to be removed.
A damaged testicle also has to be surgically removed, though I would assume you would remember if that happened.
Fortunately, the body is designed that one testicle can supply all the male hormones and sperm to father children.