Most people will probably only need one account.
The servers on the Fediverse talk to each other, which means that even people on different servers can follow each other and communicate. Think of it like your telephone: you only need one phone with one SIM card to call anyone in the world.
To put it another way:
You don’t need phones for every phone network, because the world’s phone networks talk to each other.
You don’t need email accounts on every email provider, because the world’s email providers talk to each other.
You don’t need an account for every Fediverse server, because the world’s Fediverse servers talk to each other.
The reason these are all true is because phones, email and the Fediverse all operate on similarly structured federated networks.
But some people do have multiple accounts? Why?
There are specific situations where having multiple accounts is very useful, see the guide to multiple accounts for more info.