Adults can have up to 32 teeth. The wisdom teeth are the last to come through, right at the back. They usually appear when you are between 17 and 25, although sometimes they appear many years later.
Nowadays people often have jaws that are too small for all 32 teeth – 28 is often the most we have room for. So if all the other teeth are present and healthy there may not be enough space for the wisdom teeth to come through properly.