The best way to answer this question is to install the EWWW Image Optimizer Cloud plugin and have it tell you how many images are in your Media Library. Once you have it installed, go to Media->Bulk Optimize. The next step is to hit the Scan button to find all the unoptimized images on your site. By default, it 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, and on larger sites it might take a little while to compile a complete list. But once the scan finishes, the page will display how many images the plugin has found on your site.
EWWW is the first Image Optimizer to let you optimize every single image in your WordPress site, even ones from plugins that don’t use the Media Library. On the reverse side of that, EWWW I.O. is also one of the few that will let you disable resizes that you don’t want optimized. At this point, write down the total number of unoptimized images, and head back over to Plans & Pricing 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.