Not exactly, it's a valid question because it's not really obvious what actions cause an increase in resource usage. How much data is generated by federating with an instance? Does Lemmy mirror any community a user subscribes to? For how long is data from other instances kept? Does the instance commenting or the instance hosting the post store comments under that post? I don't know how Lemmy works in that regard, it might be that a million users can use a raspi if they're only subscribed to the same few communities.

Absolutely. Ideally this would also be implemented on a backend level. An easy way to create multi sublemmies and share and Link to them would make it possible for massive communities to form on and migrate to Lemmy in a sustainable way.