You can create any number of subfolders in a personal folder. To create subfolders in a shared or public folder, you need to have the required permissions. Depending on the app, there are different ways of creating folders.
How to create a new folder in the E-Mail or Files app:
Select the folder in which you want to create the new subfolder in the folder tree.
Note: Select a folder for which you have the permission to create subfolders.
Click theicon next to the folder name. Click on .
Enter a name in the Add new folder window. Click on .