You only need one account on one server to follow and interact with people from across the Fediverse. Think of it like your telephone: you only need one phone with one SIM card to call anyone in the world.
You don’t need phones for every phone network, because the world’s phone networks talk to each other.
You don’t need an account for every Fediverse server, because the world’s Fediverse servers talk to each other.
But some people do have multiple accounts? Why?
There are specific situations where having multiple accounts is very useful, see the guide here for more on this topic.