People with one kidney are expected to have a normal lifespan. That’s one of the reasons why it’s ethically reasonable to allow someone to give a kidney away (for example in a “living related donor” transplant).
Our kidneys are made up of small functional bits called “nephrons”, and each kidney contains lots more of these than we need. This is called having a high “physiological reserve”. So, one kidney functions fine. The main problem is that people with one kidney can run into problems if the one remaining kidney is damaged, because they don’t have a spare.