I am wondering that about sub saharan Africa as well. I don't know why they are not more specific, but if you look at ETA statistics for the past few years it might help you get a better idea of how many openings each country has in general.
It looks like Rwanda has had 3 ETA positions the past 3 years so it is probable they will have around 3 again this year. It is unlikely (although not impossible) it will jump from 3 to 15 or something like that. South Africa has had 10,10,14....Here is the list. Hope it helps
http://us.fulbrightonline.org/eta-grant-application-statistics