Why are you asking? Some people are born with three, or only one, or one or two partial incompletely formed ones which may still offer them adequate renal function to live a long and healthy life. Some people lose one due to injury or disease. Some people donate one. (SUPERHEROES!!!) The kidney that is left grows in size to compensate. There are always anomalous people out there on the planet.
Chronic Kidney Disease is increasing in OECD countries (while genes definitely play a part, the increase is largely due to Diabetes and Obesity-related diseases, but Alcohol and Drug abuse are also a factor.) The medical costs are enormous and dialysis patients find the lifestyle debilitating physically and mentally. As the disease progresses it affects every aspect of their lives and every person in it.