Im not sure what you looking for. The growth/decline of a group like JW depends on scandals, teachings and much more. So I don't think that it make much sense to collect the data.
But if you need numbers, maybe this is interesting, only germany:
From 2007-2015:
- they merged 276 congs to 138 congs
- they build 116 language-congs
- build 5 new congs
- build signlanguage-congs
I think main part is to find, how they fake the statistic. The language-cong save the statistic, but mostly there not much new people.
So if you start to calculate, you base it on the 8 Mill witnesses, first you have to start there and find some Information if 8 Mill is nearly correct.