I guess that depends on if the federated users receive content via a push or poll method, and I’m not sure how that’s done in Lemmy. If the subscribing user is just requesting content from the home base and pushing their comments/posts up to it then even the home server wouldn’t nessesarily know the user count, just the number of instances polling it. From what I’ve seen trying to clean out communities ( ran a subscriber bot too long and blew up my DB to obnoxious levels ) though is if the home server thinks there is anyone subscribed though it will still look to push content to the remote end, so based on that I would think the home server should have an accurate-ish count of the total users.