People from different instances are not able to login on your instance. For example you might see me in the people table, but local should be false. People registered on your instance shows up on local_users and on people with local set to true.
There have been mass registrations going on recently. Imagine my surprise when my 3 people instance had 460 users but no one online today. Took my instance down to investigate and deleted all local users in person table except the known users. Not the cleanest way to do it but I don’t think I broke anything :D
I don’t have a straight up list to offer you, but https://lemmy.dbzer0.com/u/db0 has made a software for looking up instances with high amount of users but low amount of posts called lemmy-overseer. https://github.com/db0/lemmy-overseer
On github you can find link to their running instance with API documentation.
I have not personally used it, so this is as far as I can point you :)