Scalability - Performance vs User Base

We are preparing for launch of our app that has been in development for the last 18 months. It would be helpful if others who have done so could offer some advice about thier experience.

We do understand that server performance/DB performance ect… is highly dependant on the app and the overall archictecture - so just looking for general insights. We also realize the indexing could be an issue with large table searches since the DB indexing is not exposed and hence we do not know if the tables are actually experiencing sequencial searches, rather than indexed binary searches?

What has been your experience with user bases of 10K, 100K, 1M, ect…
Our assumption is that bubble can easily handle 10K or less?