I am new to Lemmy, but from what I can tell you can create an arbitrary community, as long as the server allows it. Same name on different instances are treated as totally separate entities. In my opinion, as a new user, I think that is highly non optimal as it creates a fragmented set of users for a given topic. If you go to feedit.de and search for technology you will see a number (seems about 10 or so) different communities with the exact same name. It is up to you to go to each one of them and figure out which one, or ones, you want to follow.
From what I see, Lemmy is growing and it may, eventually, grow if you create a local community for a city / region. My advice for anyone that would like to create a new community is to ensure you are creating content on a regular bases until there are enough other people to also contribute, with the knowledge could be a while (weeks / months???) before others discover it.