Musk: We need one million people on Mars

Why go to Mars? 

Elon Musk says that his super-ambitious effort is all about keeping the human species alive, in the event of a disaster that could lead to our extinction. 

To establish a civilization on Mars, he would like one million people to travel there. Given that people can only go every two years, with 100 people per trip, we will need to create at least a thousand ships making many trips.