More than half, according to this article from the Wall Street Journal.
Some 620 million people across India defecate outside, the largest number world-wide. About 70% of rural Indians don't use toilets, and 28 million children have no toilet facilities in school, according to Unicef. It is common practice for India's mothers to dispose of their children's waste in the open.
Open defecation is a serious public-health problem. It can expose people to diseases such as polio, giardiasis, hepatitis A and infectious diarrhea. In 2012, nearly a quarter of all young children who died of diarrhea world-wide were Indian. Constant exposure to fecal germs can also lead to stunted growth, a condition afflicting some 61 million Indian children.
India has made progress: The percentage of Indians using toilets has increased substantially since 1990, when 75% of the population defecated in the open.
There has also been a significant increase in the proportion of schools with a usable toilet, from roughly 47% in 2010 to almost 63% in 2013—and a large majority of schools now have separate toilet facilities for girls and boys.
But India is still far behind other developing countries. Neighboring Bangladesh, which is far poorer, has reduced the number of households without access to toilets to below 10%. In China, the number is about 1%.
Here's a sample of the public service campaign, Poo2Loo: