There are a lot of servers, I am on lemm.ee for example. Most are federated with each other so you can see and interact with every post from every community, so usually it doesn't matter which one you sign up with.

Sometimes a server gets maintenance issues or makes moderation decisions that make users want to move, and portable accounts aren't a thing yet in the Lemmy code, so some people have accounts on more than one server. Portable accounts is a top requested feature, so hopefully the dev implements it in a near-future update.