How many different races in the world?

There are three major races of human beings: Caucasian, Negroid, Oriental.

Answer:

Racial differentiation of the human race is a contentious issue. Some maintain that there are three (primarily based on skin colour) others one (as all human groups can interbreed. The study at the link indicates nearly a hundred (depending how you count subgroups). Compounding this is that the human races (however many thre may be) are constantly intermarrying producing children with mixed attributes set up for "pure" racial streotypes.

Even the old three races grouping based on "obvious" physical traits runs into problems. As an example the dark skinned Aborigines of Australia were lumped in with dark skinned natives of Africa ... but genetic and anthropological evidence puts them as having closer ties with Asia.

Even if it assumed that all humans ultimately originated from Africa, there were several major migrations from that Continent far enough apart that the modern descendants of each exodus were contributors to different "races"

There is little genetic difference between humans. Even on crime investigation dramas no one suggests running DNA analysis to determine race. They look for "typical" bone features and limb lengths to narrow down any racially defined description.