There are absolutely not 2.8 million active subreddits. I just spent like an hour trying to find data on this. Nobody cites their sources. I used a dump of subreddit statistics from 2018, when there were just over a million subreddits. (Source: https://www.reddit.com/r/ListOfSubreddits/comments/8gzmmv/i_created_a_better_csv_textspreadsheet_list_of/)

There were ~34,000 subreddits with more than a 1000 subscribers. And 100,000 subreddits with more than 125 subscribers.

Looking at https://subredditstats.com/ the top 5000 subreddits make up about 30% (based on an estimated 840,000 posts a day by some reddit user on a subreddit that's currently dark so I can't give a good link) of the daily posts and surely far more than 30% of the daily traffic.