Looking at the Joshua Project's catalog of people groups, here's two different views. The first is by JP's own internal levels. These are based on % Evangelical (Ev) and % Christian of all kinds (C).
Careful note: these are the populations of the groups at the stage. Within each of these levels are varying numbers of believers. For example, I estimate based on JP data that Level 1 (Unreached) contains some 16 million Christians and 5 million evangelicals. But, as we might expect, the vast majority of believers and evangelicals are found at Levels 4 and 5.
Now, by contrast, I'm experimenting with a different approach these days, based in part on some of Hans Rosling's (posthumously published) work in Factfulness. Using 5 stages based on % Christian of all kinds:
I use this latter appraoch because it's somewhat simpler and relies on a doubling of the percentages, which lends itself well to tracking exponential growth.
It is interesting to me that in either approach, there is a 'dip' between the 'very unreached' (Level 1 for JP, and Stage 0/1 for my system) and the 'more reached' (level 4/5 for JP, and Stage 3 and up for me).
In the JP system, Level 2 (Ev<2%, C>5%) is the smallest, with a population of 294 million. In mine, Stage 2 (2% to 8% C) is the smallest by far, with a population of 374 million.
What to make of this? One theory of mine: once a people group has the gospel, it tends to progress rapidly to higher levels of % Christian. Another theory: we put a lot of work into groups that have the gospel, to 'bring them up' to higher levels of % Christian, while Stage 0/1 languish.