SEOmoz had a post from 2010 that shows how to find that number in your Google Analytics. I've taken that a step further and created a custom report that does something similar:
Just click the link for Google / Organic (or whichever search engine you're interested in) and you'll then see the following which shows the number of URLs indexed that brought you traffic:
Download the Custom Report for Landing Pages Being Indexed That Bring Traffic.







