Keep in mind this is 80% of the subreddits that announced would be participating in the event. There are over 3.5 million subreddits altogether, though of course most of them are dead or with a single user.
One however also has to keep in mind that on Reddit you need either a tight-knit topic specific community, or a lot of users to have an active subreddit.
Just randomly clicking around I see that subreddits with ~50k users already qualify for Reddits top 5% largest subreddits.
If you randomly browse subreddits of that size you will notice that for many of them the newest threads are at least days old. So in reality the amount of active subreddits, let's say subreddits that get at least one thread per day or more, is probably in the realm of a few thousand.
TIL. I thought reddit had already cracked down on unmoderated subreddits; that if a subreddit didn't have active mods whom the admins could command that they would shut it down.
Just randomly clicking around I see that subreddits with ~50k users already qualify for Reddits top 5% largest subreddits.
If you randomly browse subreddits of that size you will notice that for many of them the newest threads are at least days old. So in reality the amount of active subreddits, let's say subreddits that get at least one thread per day or more, is probably in the realm of a few thousand.