There are no rules. A client, relay, or image host may have them, but Nostr as a whole does not. You can move to another client or another relay or another image host if you need to and all of your followers and notes just show up. For example, if you're on Damas, go check out Primal (TestFlight), or Plebstr, or NOS social, all of your notes and contacts will be there even though you're using a new app. It's because they all use the same protocol. Kind of like email.