To answer your question seriously: Not very many.

The most populous sub had 39,717 subscribers.

Many of those are bots. How many? Unknown.

I believe, without evidence, that many of those actual humans were one-time signups that are now gone. Reddit subreddit bans come in waves. Each time a big wave happens, it results in a wave of signups here out of a response of frustration and anger. Those people appear to sign up, look around a little, and then slowly trickle away when they realize that the site just really isn't that large.

You'd have to judge actual numbers on site engagement. Using the 90/9/1 rule of thumb, 90% of users are lurkers. 9% create an account and upvote and downvote and comment. 1% are content creators and submit new posts. So if you go by the front page, you have some posts that do not have any comments, just a few upvotes, and some posts make it to the front page without any upvotes at all.

So not very many active users. Using this post as an example, you have 16 comments and 16 upvotes. Using the 90/9/1 rule, around 160 people saw this post and 16 interacted with it. And this was one of the more active posts on the front page.