There's no hard limit imposed by the plugin itself. In practice, performance is bound by two things: the cloud provider's API rate limits, and the time WordPress takes to render very large folders.
What you can expect
- Small to medium folders (up to a few thousand files): the plugin handles these without trouble. File listings load in seconds.
- Very large folders (tens of thousands of files): still functional, but page loads slow down.
- Deep folder structures: no depth limit. Navigation is lazy-loaded, so only the current folder's contents are fetched at any time.
If performance is slow with large folders
- Reduce the number of items shown per page by splitting the folder contents into sub folders
- For the Gallery module on Box or Dropbox with many images, be aware that thumbnails are generated server-side (Google Drive and OneDrive deliver them via API, so this doesn't apply there).