Have a look at this post, which will help you find some communities to join: https://lemmy.blahaj.zone/post/61827

In short...

• Your account (and every account) has a home instance. Both you and I have our accounts on lemmy.world.
• And every community (aka subreddit) also has a home instance. The home instance for this community happens to be also be lemmy.world, same as our accounts.
• But through federation, the posts from each community gets copied from the communities home server onto each subscribed user's home server. So you can subscribe to any community on any server that lemmy.world federates with.

So while it's possible for multiple instances to have an nfl community, it's not necessary since you can sub to the NFL community on another instance and that's totally normal and expected. Think of it as the subreddits name includes its instance name, so [[email protected]](/c/[email protected]) is just a different subreddit than [[email protected]](/c/[email protected]), just like on Reddit you could have competing subs named /r/nfl and /r/nflfootball. And just like reddit, when things start to calm down, I think you will see that in cases where a bunch of dupe subs exist... one or two with active mods start to dominate on user count and those end up being the most interesting. It will be a bit wild west for a while though. Lemmy is a lot smaller than reddit, though, when I find dupes of a topic I care about, my strategy had been to subscribe to them all and I'll cancel the ones that flopped a few months from now when it's clear what is active and what's dead.