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, you can create folders or subfolders.
How to create a new subfolder 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.
Enter a name in the New subfolder window. Click on .