Percentage of screen readers users in USA?

"Percentage of screen readers users in USA" : Is there a place where I can find such statistics ?

A more global statistic that would still be good too.

I came across this page but I could not find the more "global" numbers I was looking for: http://webaim.org/projects/screenreadersurvey5/

EDIT

Here I am not trying to discuss whether or not I should make my website screen reader friendly.

Please stick to answering to the title of the question. If you want to answer whether people should do their website for screen readers or not then create a new question on this forum.