Q. What is the population of unreached people and the number of people groups in India?
This is probably one of the most challenging (and even contentious) issues to answer from a missionary research standpoint. In fact, when I’m teaching Perspectives or talking with various people, I often point out that the various global lists (World Christian Database, JP, IMB, Wycliffe) are very similar, with just a few differences here and there–the massive exception is how they handle India’s peoples and castes.
Ethnologue lists 461 languages for India, of which 447 are living. Joshua Project lists 2,157 people groups, of which 1,948 are “unreached.” IMB lists 1,154 people groups. The World Christian Database lists 482 people groups.
The WCD uses a mostly linguistic viewpoint (e.g. Eastern Hindi, Bangri, Bagri, Central Bhil, Eastern Bhil, Northern Bhil, etc): their 482 peoples represent 465 languages, matching almost one-for-one with those listed in the Ethnologue (Bangri matches to code ‘bgc’, for example, but the three Bhil groups listed above all correspond to one language code ‘bhb’ in the Ethnologue).
Joshua Project relies on the OMID data set for India, and breaks the peoples down further into castes. The population of the groups they mark as unreached is 1.21 billion (out of 1.28 billion, or pretty much all of India).
The IMB takes a midpoint. Jim Haney, Director of Research for IMB, told me in an email:

A couple of years ago Luis Bush and I wrote an article about segmenting people groups in South Asia. We based that article on Matthew 28:19 and a biblical understanding of ethne as a descriptor of people groups in the New Testament. We agreed the primary paradigm for people groups in South Asia is to look at them through castes and communities. However, many Muslim people groups continue to be viewed best ethnolinguisticly. With that said, IMB depends on our observations of the way people actually gather when the gospel is communicated on the ground. In other words, segmentation cannot be pre-determined. First, the church planter shares the gospel. Then, we observe who will come to the church. When we find certain people do not come, we see that people as a candidate people group. This is very similar to the way SIL treats a new speech variety. At the point of discovery, it’s not clear if the speech variety is a language or dialect. In the same way, when we find a people group that faces a barrier to understanding or accepting the gospel, we’re likely to create a people group identity so the gospel can be communicated through a separate effort to that people group…

So the result is that the number of unreached people groups varies according to the methodology. But the result is pretty much the same: no matter which of the three primary people group ways of measuring, India is pretty heavily unevangelized.