As a general rule of thumb most mammals have two sets during their lifetime, first baby teeth, which shed naturally, then a permanent adult set. If those adult teeth are damaged or wear out, a mammal can't eat - and eventually dies. Although there are exceptions, elephants have six sets and dolphins only have one. Fish, amphibians and most reptiles grow new teeth all the time, as soon as a tooth is lost a new one starts growing. A crocodile can have more than 50 complete sets of teeth in a lifetime.