Explain the note in README about scaling

The README file says:

> It only works if you have a single instance of your server and doesn't scale beyond a couple of connections.

I understand the first part of this sentence - only one server can listen for publish requests - so no horizontal scaling. However what does the second part mean? What limits the implementation to a couple of connections? Do you literally mean two?

Start with the scaling note in README and inspect the subscription implementation it describes. Determine what “a couple of connections” means and update the README so the connection limit and scaling behavior are explicit.

Written by the indexing model from the issue text.