when do you consider scaling horizontally versus vertically

So, the app's getting pretty popular, and I'm thinking about how to handle the extra traffic. My first thought is just adding more servers, you know, horizontal scaling. But when does that stop making sense? Like, when is it actually better, or even necessary, to just get bigger machines (vertical scaling) instead? Are there specific situations or numbers that make you choose one over the other? I'd love to hear about what worked for others and what the downsides are.