The number that matters isn't really what dents Reddit, but what gives Lemmy (or the fediverse as a whole) the critical mass to actually become viable.
27k isn't yet there. If I had to guess, I'd say maybe 200k is where you need to get to. You need niche topics of interest to say, 5% of the population to get communities of at least 10 active people who will post, comment and curate. If only 10% of people do that, you have to have a user base such that 0.5% of it can sustain an interesting content.
However it isn't uniform, so perhaps 27k is already enough for the very largest communities to get off the ground.
27k isn't yet there. If I had to guess, I'd say maybe 200k is where you need to get to. You need niche topics of interest to say, 5% of the population to get communities of at least 10 active people who will post, comment and curate. If only 10% of people do that, you have to have a user base such that 0.5% of it can sustain an interesting content.
However it isn't uniform, so perhaps 27k is already enough for the very largest communities to get off the ground.