Because of the way UrBackup does file de-duplication to save space and time, almost all utilities cannot provide an accurate count of the space used. The only certain way is to see the total used and free space of the volume, which does not depend on counting individual files. Either view This PC in File Explorer and select View by Tiles, which shows a bar with the free space remaining, or right-click on the volume (your 8TB drive) and pick Properties. The amount of Used space, Free space, and Capacity will tell you how much of that storage is currently used by backups and other files.
If you drill down in File Explorer to a single backup within your storage volume, for example ClientName\210724-0840, the Properties will tell you approximately how much space would be used to restore all the files in all the folders that were marked for backup at that point in time.
This is not the amount of data transferred from the Client to the Server during that backup, nor is it the amount of storage used in addition to previous backups for that Client. That information is found on the Activities tab in the web interface for the Server.
UrBackup does its best to transfer and save each file only once, unlike most other backup programs. It also manages the storage to keep the maximum number of backups for each client by deleting older backups only when necessary. This means the normal state of an UrBackup storage volume is “nearly full”. You may want to set the Global soft filesystem quota to a smaller percentage (such as 80%) if your 8TB drive is shared with other programs. This will tell UrBackup to leave more free space on the volume after nightly maintenance.