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/
It will change things too, not for reddit, but for competitors (like kbin).
A tiny site can only grow so fast, at some point things start breaking (both technically and as a community) and users stop joining, but as sites grow bigger they also gain the ability to grow faster.
The protest means that every possible alternative to reddit has been growing as fast as it can reasonably support. That’s probably not fast enough to hurt reddit this time, but next time it might be.