How many teeth does an adult human usually have?

Thirty-two, including four wisdom teeth - and a large and growing share of people never get all four, or any. Somewhere between a fifth and a third of people are missing at least one, with the proportion varying sharply by ancestry, and that appears to be evolution in progress rather than a defect.

The reason they cause trouble is that human jaws have shortened. Cooked and processed food requires far less chewing, jaws develop smaller than they did in populations eating tougher diets, and the teeth arrive to find the room gone. Skulls from hunter-gatherer populations show far less crowding and impaction than modern ones, which suggests the problem is developmental rather than genetic.

Children have twenty milk teeth, which they lose, and that is unusual: most vertebrates either keep replacing teeth for life - sharks and crocodiles run a conveyor - or grow one set. Mammals got two because a mammal's teeth have to occlude precisely against each other to chew, and precise occlusion cannot survive continuous replacement. We trade the ability to regrow a tooth for the ability to chew properly, which is a bargain that felt reasonable before dentistry.