There are no requirements, and they wouldn’t be enforceable even if somebody tried. The admin of instance1 has no way of knowing that you already have an account on instance2. Your identifiable details (IP address, e-mail address) are private to the instance that you sign up with and it would be a violation of privacy (and inherently scummy) for those to be shared between instances - they’re not.

You can be anonymous on the fediverse, just like the Internet in general used to be before Facebook.

Not at all. You don’t have to identify as yourself on every instance you have an account. It’s literally a different account. You can keep a backup of all your subscriptions/follows/etc from one to the other or you can treat them as two entirely different identities. Just don’t have them generate a lot of pointless traffic between each other (like arguing with yourself on a post or something).

each instance has its own user base, charlie@instance and charlie@otherinstance are not guaranteed to be the same person any more than @charlie at twitville and @charlie at ig.

In the Fediverse it isn’t possible to reserve your username. I could host a server and get something like favrion@lemmy.account so it’s unreasonable to expect that users keep the same username across instances. Reddit is more anonymous than Facebook. The Fediverse is more anonymous than Reddit.