Buddhism essentially arose as a faith as a direct response to the inequalities of the caste system, historically attracting large number of converts from among the Sudras and Dalits.
Histpry has it that under Emperor Ashoka, Buddhism was the predominant religion in india, but after his death and the subsequent Hindu revival, large scale massacres of Buddhists and destruction of Stupas took place in India, decimating the faith.
Today, the Buddhist population is very small in comparison to the population of the other minority faiths and most of the present day Buddhist population compriuses of Dalit and other low caste converts.
In the recent years, Dalit intellectuals and leaders have embarked on a campaign to convert the Dalits to Buddhism in order to escape caste.
Regularly, conversions of Dalits to Buddhism takes place in India, but there are no specific statistics about the numbers.
NOTE: You can get very detailed and extensive information about the conversion of Dalits to non-casteist faiths including to Christianity and Buddhism in the eBook 'The Truth About Dalits'.