...also:

-You only need to sign up on one server. The servers communicate to form a single network. People from different servers can follow and interact with each other.

-Don't sign up on mastodon.social, it's way too overloaded. You will have a nicer experience on a different server. You can find well-run reliable servers at https://joinmastodon.org/servers and https://fedi.garden

-You can always move and keep your followers/follows if you ever change your mind about being on a particular server