Just say 40,000. Which is a pathetic number, but perfectly fine for the type of niche communities budding up here and there across all the domains connected together here.
40000 is enough to be a functioning social media. most fediverse softwares don't have that much. Sure, it is not enough to have discussions over non mainstream stuff, but there are still enough people for a variety of topics.
There are huge subreddits that are basically dead or just filled with spam. The ratio of active/passive users on Lemmy must be much much larger. A Lemmy community with 100 active members almost feels like a subreddit with 10 000 members.
A Lemmy community with 100 active members is more likely to be 100 active humans than a subreddit with 10,000 members is, based on the last time I went to Reddit: it was so, so clear that everything was either ChatGPT, or a repost of shit even I had already seen, or was just otherwise obviously not an authentic human sharing something interesting.
It might also be that we were some of the prolific posters on reddit. I heard somewhere that the top couple percent of posters on reddit used to make a majority of the new posts. And the rest lurk
That's probably true, though I'm not sure who has ever actually made a legitimate determination since you'd have to remove the non-humans from the numbers first and, well, Reddit isn't going to tank their MAU numbers by ever releasing that kind of stat.
It's also not helped once you hit a certain size and the nature of scale takes over and the level of toxicity goes up: even in small groups, when a new person shows up and asks the same question for the 20th time, they start taking shit for it. If you're in a BIG group, it turns into a giant dogpile, and people stop asking questions because who the hell likes that kind of response, so you end up with a lot of people who are subscribed to something, but none of whom actually contribute at all.