The total number of people groups in the world varies by which list you are looking at. The number of groups that are unreached varies in the same way, depending on definitions. And trying to find a group to focus on amongst 16,000 or so can be overwhelming.
A far easier and perhaps more strategic route is to look at clusters. While there are tens of thousands of groups, there are only 250 or so people "clusters."
Clusters aren't really a precise or technical definition. As far as I've been able to determine, they were first advocated by Patrick Johnstone. There are two "cluster" methodologies - one is used by the World Christian Encyclopedia and the other by Joshua Project. They are so close as makes very little difference which one you use. The WCE one is less accessible, being in a print book and a subscriber-only database, so I generally suggest the Joshua Project one purely for ease of access.
By focusing on a "cluster" you are looking at a larger "family of peoples" that generally share cultural, economic, political and linguistic situations. There will still be some minor differences, but it's probably going to be easier to get the Gospel to cascade between peoples within a cluster than between clusters.
Strategically speaking, I'd love to see every people CLUSTER "adopted" by a church. In writing this, I realize it goes against the "specialization vs generalization" argument I made last week. On the other hand, it would bring balance to the situation. Alternatively, it would be great to see small-and-medium sized agencies or networks pop up that would focus on one of these clusters. Several of the clusters already have networks, others need them.
If you or someone you know is thinking about a people group focus, why not print out the cluster list and suggest it to them? Or, if you're a church, why not print out the people group list and see about identifying which groups have networks and which don't, and how you might help push things along?
If nothing else, check out my index to prayer guides (which uses a Cluster based approach), find a cluster lacking a prayer guide, and see what you can do about getting one authored.