It can't scale without spinning up new (heavy) instance. Its database is hosted in the same <<thing>> as the web server (
https://join-lemmy.org/docs/en/administration/install_docker... ). Note that it's a single container that exposes port 80 and 443 - the database is hosted inside of that container.
That makes scaling the main instance impractical without redesigning how its built, deployed, secured, and connects to its backend database.
Note also that adding other sites increases this load without adding users as polling for content constantly is likely more expensive than having a user connect directly (and use it for a limited time).