Generally most people only need one account on the Fediverse, because the servers talk to each other as if they were on a single network.
However, in some situations and for some purposes you may prefer to use multiple accounts. If you're one of these people, there's a guide to using multiple accounts here:
https://fedi.tips/using-multiple-accounts-on-mastodon-and-the-fediverse
Also see the article on why most people don't need multiple accounts:
https://fedi.tips/do-i-need-multiple-accounts-do-i-need-to-join-lots-of-servers
