When do you introduce horizontal scaling in an Elixir/Phoenix app?

I created an app that uses Phoenix LiveView and is deployed on Gigalixir. The app manages the state of multiple players (for example, a poker table) using a Registry and a GenServer. I’m aware that Phoenix can handle many simultaneous connections, but my concern is that if I ever want to scale horizontally, I will need to use something like a Global Registry and libcluster; otherwise, the state might be split between multiple replicas. I’ve read some conflicting advice online: either that, for a start-up using Phoenix LiveView, it’s fine to start with a single replica and the decision of scaling should be left until the app actually starts receiving heavy traffic, or that it’s easier to start building a scalable app from the beginning rather than adjusting for scale later on.

So, do I wait until the traffic load becomes a strain, or do I try to implement horizontal scaling from the start? I have no idea how many users I’m going to have. I will be hosting a poker tournament with a $100 USD giveaway, so I would expect some traffic, but not very much. I gave myself about a month to figure out this problem and work out any bugs in the app before the tournament begins. Any guidance on this is greatly appreciated. You can see the site at High Society.