Welcome to the community! I’m not sure how many users Adalo can handle, but here’s a screenshot of the users’ database showing the current number of users. It’s working fine, and I’ve seen multiple apps with more than this many users, so I believe Adalo can handle a good amount of users.
It’s the best way to get your idea out of your head and into a functional app without a steep learning curve. The limitations only become an issue when your app becomes very successful and large, and by then, you’ll have revenue and user validation to justify the cost or learning curve of a more scalable, advanced solution.