As of 12:56pm GMT (7:56am central time), 7742/8299 subreddits are no longer public
The information I initially posted is misleading. Thus, I have edited the title and this content area to report accurate numbers.
As @roofuskit mentioned, the 8299 number is the amount of subreddits that committed to going dark, not the total number of subreddits, which is over 3,000,000.
And as @8thiest was keen to observe, 204 of the top 250 are dark, as you can see from this site: https://save3rdpartyapps.com/
This isn’t going to completely destroy reddit. Things will eventually go back to mostly how they were, but the difference is that some other places will have attracted a critical mass of people so as to provide an actual alternative. What’s probably going to happen is that reddit will begin a long, slow decline over the course of years. It’ll probably be around in some form or another for quite a long time, much in the same way that Slashdot, Fark, and Digg are still sputtering along.