In the real world, the definition of scalability may vary from culture to culture, but for your app it is important that it is responsive and functional in the most common circumstances.
If you foresee any issues at a given moment in time, it is time to scale up; but the key element here is that being able to scale up quickly is more important. Make sure that you can do the right things when there is momentum!
So, you made an app and it has been shown on a site such as Product Hunt or Betalist. You have some enthusiastic early adopters for an audience. As an early-stage startup company, you should not care too much about how well your app scales, but you should have an idea on how to make it scale if your audience suddenly becomes larger than expected.
Scalability is not just about the backend solution for your app. It is also about...