The best way to answer this question is to go to Media->Bulk Optimize and have the plugin count how many images are in your Media Library. The first phase just scans your library, and even without an API key, it will check all your images to see how many there are to optimize.
*By default, EWWW IO scans the Media Library, your theme, BuddyPress, and a couple other folders for pre-configured plugins.
Depending on your site, it might take just a second or two, while on larger sites it might take a couple minutes to compile a complete list. Once the scan finishes, the page will display how many images the plugin has found on your site. At this point, write down the total number of un-optimized images, and head back over to the Pricing page to see how much that will cost.
If you want to disable a few of the resizes being generated to save money, be sure you know which ones are being used before you get too far. Or, you can sign-up for an Unlimited Plan and stop worrying about how many images need to be optimized!