From https://fediverse.observer/stats
Which seems to not at all come close to representing what you might actually see on Lemmy, not that Lemmy is tiny either.
Has there been any attempt to measure the total active Lemmy userbase? This would depend on the definition of Active but any working definition would be more useful than counting account creation. Something like "posted at least once this quarter".
The 2m there is extremely inflated to to bot sign up spam. Back when instances were getting set up in the reddit migration there was some people that started mass signing up accounts on instances with weak security. Fediverse observer doesnt care about any of that and just shows user accounts regardless of what happens on the instance
Fedidb has a much more accurate count at ~ 400k https://fedidb.org/software/lemmy since they exclude these bot spammed instances. They also have a monthly active user count (~ 38k) based on what instances report for that stat