After you run the backup tests, please use your operating system’s “disk free space” command (df on Linux, volume Properties on Windows) to see how much of your 11Tb remains. Most “disk used” counts seriously overstate the actual space occupied, as UrBackup also creates a directory for each backup that links the files in the common storage area into a pseudo-full-backup which can be copied or shared by normal file operations. Yes, even incrementals get this point-in-time pseudo-backup with links to all of the files, whether already known or freshly added. (Look in your /urbackup_data directory under each client name.)
In some cases each file on that server being backed up might be counted many times - once for the common file storage area and again for each full or incremental pseudo-backup directory. For example, I have a 4TB storage drive that Windows counts as having 49.8TB used in 34,586,071 Files, 3,227,904 Folders. Counting that with Folder Properties took two hours.