Nearly one billion.
A slum is a densely populated settlement in an urban area which lacks basic amenities like sanitation, potable drinking water, electricity supply etc. Slums are crudely built settlements and they may shanties or dilapidated structures.
According to a United Nations Habitat report, nearly one billion people live in slums today- that is about one in every six human beings. And this figure will be doubled in the next thirty years. As per their 2012 publication, approximately 33% of the urban people living in developing countries (around 863 million) live in slums.