Welcome Francesco!
I think the most recent response to this question is this one; while it comes up all the time, I agree it doesn’t appear in your face when doing a search. If your analysis is restricted to defining voxel masks for specific anatomical pathways from which to sample voxel-wise quantitative values, then @Lee_Reid 's manuscript is highly relevant; you may be looking to use editing of a whole-brain tractogram rather than targeted tracking, and not interested in quantitative volume so much as the results of sampling those metrics, but much of the logic presented in that article nevertheless applies.
My own intuition is that 100-200k whole-brain is insufficient even for that kind of quantification, as the number of streamlines within segmented bundles is going to be very small. But I’m also the one who set the precedent for 100m streamlines so might be a little biased… 
Hopefully we’ll finally have a more authoritative answer to this pragmatic question in the not too distant future. Until then, my very generalised answer remains the same: devise a test as objective as possible that looks at the variance of your own experimental results as a function of streamline count.
Cheers
Rob