About increase in users

Is there any case that the app may not work stably when the number of its user accounts exceed a certain number? If there is such limitation, what is that number approximately?

Only one user is using the app at a time on each device, and each device has its own copy of the data, so the number of concurrent users in no way affects the behavior of the app on any given device.

Performance problems, when they do occur, are entirely a function of the size of your data and the complexity of your app.