No person has ever 'explored' any planet, besides Earth. Several people have been to the moon. Unmanned vehicles have been to Mars. Mankind has only 'explored' other planets visually.
This is a matter of opinion as to what is too much. Personally I think that there probably are too many people and this is the root cause of our contribution to climate change. If there were fewer...
Think about it. A billion in china, a billion in India, Canada and USA make a billion, and the rest of the world is probably 2 billion.... So 5 billion? More?