Yep, it’s how many are subscribed from your instance.

You need to be on a web browser, and at the instance you want to check. Scroll all the way to the bottom, and click instances. That example is from your home instance.

I’ve used Plex, Jellyfin, and Navidrome. I’m currently using Navidrome, but Plexamp is a massively better product for discovering/interacting with your library. The sonic analysis really takes it to another level.