You can choose whether or not your instance is federated - you can also choose if you want to only allow specific instances or block specific instances. Once you start interacting, for example subscribing to other instances, the number of servers federated with your server will build up.

Here’s a bit more about how that works: https://join-lemmy.org/docs/en/administration/federation_getting_started.html

By the way, this question would be better off in !lemmy_support@lemmy.ml