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).
keep in mind thats active users. that number is usually a small percentage of unique visitors in any system.
If I recall, the rule of thumb is orders of magnitude. If you have 100 active users, there are probably about 1,000 lurkers, and only 10 substantial users that account for most of your content.
This is the rule from corporate marketing stats, tho, so who knows if the Fedi works the same.