Hindi and Bengali speakers are the least multilingual. It must be noted that they are also the largest linguistic groups.
According to a report published in Time of India, surprisingly those who speak Urdu as their mother tongue are the most multilingual with 62% knowing one more language, typically Hindi.
The second most multilingual are Punjabis, 53% of them know two languages. Marathi speakers are in the third position, 47% of them speak one more language. Only 12% of the 520 million Hindi speakers were bilingual. Whereas among 97 million Bengali speakers, just 18%, or 17 million, were bilingual.