There is no way of providing an exact percentage of websites that are embeddable (and those that are not).
Many sites on the internet do support iframes/embedding.
But actually the percentage strongly depends upon the kind of website one considers each time, thus it can go from a low percentage (0-5%) to a high one (up to 80-90%), depending on the subject.
For instance security related websites, banking portals and payment processing sites like PayPal do not allow to be embedded (iframing).
Also, several big e-commerce websites (like Amazon or Zalando for example), do not allow to iframe their pages.
On the other hand there are huge informative websites (like for example Mashable, Sports Illustrated, or the Economist... to mention a few) which allow iframing their content.
If you find a content which is not embeddable, we recommend to look for an alternative and similar content on the Internet, which is embeddable.
Please remember you can also leverage our embeddable test tool, within the dashboard (go to account -> Settings -> Tools) to test, right on, whether a webpage is embeddable or not.