Yes, it will be memory dependent. For performance reasons, CC must keep enough information about each book in memory to be able to quickly fetch what to display. That information will depend on grouping and the like. Success is also dependent on how well the iOS memory garbage collection behaves. I am not convinced the iOS is better than Android in this regard.
For Android I have numbers. Over the last month (1 Nov - today), we have seen the following logged events
We see that at least one person has a library of more than 48,000 books. CC has started 81 times on different devices with an average library size of 24,544 books. And so on.
Note: "Unique devices" is really "unique CC instances". Because we don't keep any serial numbers, IP addresses, or other unique personally identifiableinformation, a device appears to be new every time it reboots or every time CC is force-stopped.