You can find the top 50 largest subreddits by amount of subscribers here: https://subredditstats.com/list/most-subscribers
Compare it to the list of https://reddark.untone.uk/ and you can see that not all the largest subreddits are listed, for example r/AskReddit.
In march 2023 there were 3,125,000 subreddits. So the total % of subreddits going dark is very low. However I assume a lot of subreddits are very small. It would be interesting to see how many of the top 1,000 or 10,000 subreddits are in private mode right now. source for total amount of subreddits: https://backlinko.com/reddit-users
I haven't run it myself, but I found SlyFlourish's tips very helpful for Ghost of Saltmarsh. He also has videos for Wild Beyond the Witchlight: https://youtube.com/playlist?list=PLb39x-29puapE8g8vYg-DXDcgQbHltF8a