I think the usage is what really uses the resources not just the account itself

also 100 accounts being subscribed to a community is basically the same as 1 account being subscribed, it's going to federate the same data either way

To add to what others have said, the real source of load is browsing. It's much better than a few months ago, but it is still the case that users actively on the web frontend is the biggest load.

If you are actively using a single digit number of accounts simultaneously, no issue. A bot farm using 100,000 users simultaneously is a big problem.

An account is just a small database entry. It probably costs the server less than a post that appears on the front page.

Not bad. It's normal to have alts on any social media platform

Limited servers available? Can you elaborate on this?