Photoshop 18 Elements - Error Moving Media

It's not a maximum number, it's any kind of error you would meet when moving files from the explorer, particularly moving files or folders to a location creating a risk of duplication or oveerwriting. Unfortunately, the error message does not give more precise info.

If you want to move files or folders from the organizer, which is feasible but not a general necessity, you have to follow the same rules as moving from the explorer. You can move selected items from the workspace to a new subfolders or you can use the drag and drop method in the left folders panel in tree mode. In each case the destination folder should not have files or subfolders with the same name as the moved items. You can even drag a folder with its subfolders elsewhere; in that case it's safer to move to an empty destination folder to avoid duplication risks.

Remember that the organizer is not based on folders, so re-organizing the folder structure generally needs a simple structure, generally and mainly based on dates.