It is impossible to estimate the total dolphin population. There are 42 species of dolphin spanning many different regions. Some species' populations are known but no one has estimated the total for all dolphins.
Dolphins are a widely varying species and can be found in many different habitats from seas to oceans and even rivers. For example, the Indus dolphin, which is a blind dolphin species that inhabits rivers in India, has only 600 members and is considered endangered. Even though dolphins live in groups of 5 to several hundred their widespread distribution and tendency to travel great distances makes population estimates difficult.