Can someone ELI5 how lemmy instances work?

I found this infographic helpful: https://i.imgur.io/b2QuYAR_d.webp?maxwidth=640&shape=thumb&fidelity=medium

Imagine there are two Reddits, A and B. You only need one account because you can subscribe to subreddits on either. Both /r/gaming@RedditA and /r/gaming@RedditB can exist, and you can subscribe to both from anywhere, with the caveat that they are two distinct subreddits.

It'll pay to look around and find a good 'home' instance, where a good amount of quality content that you're interested in is local to that instance. It'll also help if that instance has a good number of users and is relatively stable.

The likelihood of an instance disappearing, especially right now while seemingly everyone is scrambling to spin one up, is high. My single-user Mastodon instance can suck in hundreds of gigabytes of traffic in a week. Running a busy multi-user instance is expensive, and quickly jumps from novice to in-over-your-head levels of administration. Some instances will gain just enough traction to cost too much and die quickly, while others will never see a sustaining number of users and die slowly.

Both Lemmy and the current alternate, kbin, need something like RedditMigration because, without a map of what has been established where, all of these instances are doomed to share a finite number of users spread out over a given subject matter. It'd be like trying to watch a television show where each character is on a separate channel.

You know how you can sign up for email newsletters? And how anyone regardless if it's Gmail, Outlook, Yahoo, etc can get those messages and email other people? They work very similar to that.

You sign up for an account on your preferred provider (gmail), and then you can send and receive messages/posts/comments from other people on other servers.

I've written a bit of a summary here in a related topic that you may find helpful.

To answer your specific questions:

will I need twenty accounts to follow all of the groups?

Generally speaking, you will only need one account. Instances federate (connect) with each other to share information and allow their members to post on each others' content.

However, instance administrators can choose not to federate with certain instances, for any reason. They will commonly block instances whose members cause trouble (e.g. due to lack of moderation), but sometimes the reasons can be petty or political. It's best to choose an instance whose moderators and administrators have views and political opinions align closely with your own, otherwise you may find that they block instances you want access to.

What is the likelihoood of an instance dissapearing?

It certainly happens, but you need to assess this possibility on a case by case basis. Instances that are run by a single administrator seem most prone to disappearing - they can pull the plug at any time. I would avoid those and instead look for instances that have a team of administrators, some funding (usually donations, and you may wish to consider contributing), and a plan to keep the site running throughout the future.

How do you gauge the culture of an instance?

Check the moderation log, which is public on Lemmy - you don't even need an account to see it. Look at the bans, suspensions, and deletions in the log. Look at the rationale given to explain the moderation actions. Do you agree with them, or are they unreasonable? If you don't like the instance's moderation style, you probably won't like its culture.

Other than that, check out the posts and the people in the community. Do they tend to post content you're uncomfortable with? How about the instance rules - do you like them?

Are there any gotchas from joining an instance that I should be aware of?

Remember the administrators (and potentially moderators) can see anything you post, including private messages. Be careful what you share. Try to pick instance with a team you can have some measure of trust in.

I hope that helps. There are tonnes of Lemmy communities now. Find one you like the look of and give it a spin. If you need to, you can always create a different account elsewhere.

will I need twenty accounts to follow all of the groups?

No, at least not necessarily.

It's up to the instance admin on what other instances they are connected to. Beehaw.org, one of the popular Lemmy instances, actually just the other day announced they were "defederating" from two other popular instances, because those two others were lacking in moderation (leading to an increase in low-quality content and more mod work for the Beehaw admins).

Content from all instances that are connected can be viewed from one another. Meaning if you sign up for instance A, and it's federated with instances B, C, and D, you can still see content posted on B, C, and D without having to create an account for those other instances.