Most apps can run fine for millions or hundreds of thousands of user sessions on a $5-$50 VPS. People prematurely optimize for scale, adding a lot of complexity that only makes development slower, and by having more moving parts, there are more things that can break. Start simple. Scaling is mostly a solved problem nowadays, if you quickly need to scale, there are always solutions to do so. In the worst case, you have to scale horizontally, and if you reach the limit of horizontal scaling, other your app is inefficient, or your business is already successful, so you are no longer in the "start" phase.
