The jerboa app does this, and it helps when searching for a community to see how many people use it, especially when there are multiple across different instances.
That said, I don’t really know how accurate their number is so maybe it’s not super viable
These numbers are super confusing right now.
I’ve spoken to tons of people who attempted to subscribe to the biggest community on a topic but got it backwards because they didn’t see the full subscriber count on remote communities and misjudged their size.
This is a useful feature, but worth noting how confusing the numbers are. Liftoff should either explain them better than Jerboa, or consider fetching user-counts from the community’s server (though this violates the spirit of federation a bit, I don’t think there’s another way to get apples to apples numbers).