For level 6, you need approximately 110K MIB's per country (2+ countries).
I personally, advise having a bit more to get the index up faster.
I don't know why everyone is so focused on having the "exact" amount, and then complaining when the index takes forever. Its because the index is computed as 4 X current index + new index, all divided by 5. This means, that it will take awhile to catch up.
For Defense Index, it is even worse, because its computed off the already lagging defensive index, and it lags itself.