How many humans live on Earth?
There are billions and billions of humans on earth.
As of Apr 24, 2011 it was just under 7 billion
They are currently trying to build a vessel that will make it capable for humans to live in severe environments but yes Earth is the only planet where humans can live without the use of some form of protection or an artificial atmosphere. But who knows maybe they will find a planet in the future that will be just as habitable as Earth is.