There used to be the Alexa ranking for websites but it seems to no exist anymore. Elsewhere I only could find for lemmy.world ranking (something like 55000, yay!) but not the actual userbase, and nothing about any other instance (let alone the entire Fediverse).
FediDB has listed the overall Lemmy monthly active users at about 45k for a while now. It’s anyone’s guess how many actual humans that represents, since the number includes bots and alt accounts.
That number only includes accounts that post, comment, or vote. I would love to know how many silent lurkers are out there.