This chart shows the total population of the countries in each of the various stages as of the years 1950, 2000 and 2050. Fluctuations in population sizes happen partially due to population growth and partially as countries moved from stage to stage during the time periods in question. (A big example of the latter is the transition of China from Stage 1 in 1950 to Stage 2 in 2000, and on to Stage 3 in 2050). The big takeaway of this graph is the growth in Stages 0 and 1 (both in the typical numerical measure of unevangelized/unreached/least-reached/etc), and Stage 2 (between 2% and 8%, much of which would be on the verge of "unreached", just "sliding out"). We have some progress, but by 2050 what most people would define as "the remaining task" will likely be on the same order of magnitude in size as it was in 2000--unless something changes.
Click for full size graph. Data sources: World Christian Trends / World Christian Encyclopedia / Operation World for % Christian estimates for 1950 - 2050, UN Population Prospects for Populations of Countries.)