We can evaluate this by considering the results of DNS blocking ThePirateBay.
ref: https://kagi.com/search?q=How+effective+was+dns+blocking+the...
Results are references. Links pasted in HN can leave some doubt what's at the other end. Usually that's fine but by presenting those same links thru a search engine, you get some text to help indicate whether they have the info you want.
The results may also include links you would want - but maybe I wouldn't choose.
> Do you have a direct source for how many Americans accessed TPB after it was DNS blocked in all of the US?
This is a very narrow scenario you've presented. It doesn't seem to address the aspects of the thread in a helpful way.