Pizz The ads are served by Google Adsense.
You can easily check what CPM various ad networks give. It maxes out at $10 CPM, and most averaging around $1 CPM,
https://www.shoutmeloud.com/5-cpm-advertising-programs-to-make-money-from-your-blog.html
That means $10 per 1000 views, or 1 cent a view. One SS costs about 2 cents, so that's a ratio of 2x. If the CPM drops to $1 per 1000 views, the ratio is 20x.
Actually, I might have made a mistake, assuming using max CPM of $10. It's more like a chapter (average 10 SS) that is equivalent to 20 ads. So yea...to 'unlock' a chapter with ads, you need to watch 20 ads. But this is assuming that repeatedly watching an ad by the same person continues paying you the same CPM.
Also, ad revenue has been declining over the years, so it will eventually not be a sustainable model