Looking at the Linked Instances, most instances have at most a few hundred instances linked. What happens when there are thousands, or possibly even millions? Could too many instances require significantly more powerful hardware, or even be impossible altogether?
It’s complicated.
You could federate with millions of instances if the people using them are not doing very much. But a handful of instances could theoretically overwhelm the whole network if they had malicious bots spamming like crazy.
The instance that hosts a community is responsible for sending copies of everything to all following instances, so most of the load is on those instances that host lots of popular communities. All other instances just need to be powerful enough to keep up with what they receive, which is 100x less work.
So even more important than spreading users out among instances, is spreading communities out among instances. Communities cause most of the load.