text: It depends entirely on how rapidly the people can increase their population. For organisms that are rapidly reproducing there isn;t any real limit to the number of individuals required. One of each will work just fine. That's because the degree of genetic diversity will be increasing simply through re-assortment, and with an increasing population mutations will be arising more than fast enough to replace any losses.\n\nThe thing we need to know is how fast you think people are going to be able to expand, and that depends entirely on where these two survivors are located. In relatively benign environments such as the eastern US or Southern India people would be able to increase their populations exponentially immediately. In those circumstances two survivors would work. In more hostile environments such as Australia or Siberia people would find life a struggle for a long time and you'd probably need a good few hundred.