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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.