I do understand that's how it works, but considering I'm on the largest instance I'm not really sure that matters for me. Definitely worth keeping in mind for people on smaller instances though, or people interested in very small communities.

For what it's worth, I am subscribed to 55 communities, from back when I was giving the Subscribed page a shot.

An option to search only communities that aren't yet federated to your instance may be an interesting feature to add in the future.