In case you’re actually curious, a bizarrely large amount. Around 4 years ago, the admins ceased banning spammers that created their own private spam subreddits, which allowed spam to proliferate and additionally made the site metrics go crazy. Spam in subreddits no one visits amounts to about 40 percent of all (posts or comments, can’t remember which) on the website. The admins delisted those subreddits from the ‘go to random subreddit’ button, so they’re acutely aware of them. If you’d like to learn more, ask some former admins.