Nah. As long as your instance is federating with all the ones that you want to interact with, there isn't much of a reason to have multiple accounts. However, if you want to see and interact with content that your main instance has blocked, you might then make another account.

You can do that, but if you don't want 20 different accounts, I would recommend signing up to just one. Lemmy and any other federated social media system should push posts from other servers to your feed depending on what servers are in region.