5tb

anarchoi wrote:
oh ok so that's why i currently have 8.13tb volume on my 4x3tb NAS I don't understand how it is possible to backup 8TB of data on only 4TB of drive

Not backup, redundancy. A backup is stored on a separate device preferably at a different location.

This is not the best illustration but you could think of it like this:

Let us call the four disks A, B, C and D

Disk A = 1/3 B + 1/3 C + 1/3 D
Disk B = 1/3 A + 1/3 C + 1/3 D
Disk C = 1/3 A + 1/3 B + 1/3 D
Disk D = 1/3 A + 1/3 B + 1/3 C

If any one disk fails your data remains intact as between your other disks you have a copy of all the data on the failed disk.

anarchoi wrote:
I suppose if two drives fails then i am totally screwed ?

You would need to try cloning one of the disks but if the disks are too far gone then you would need to restore data from backup.

anarchoi wrote:
Even if only 4TB is used to "backup" 8TB of data, do i still get the same data protection than if i kept a full duplicate of my datas (50% disk space for files and 50% for backup) ?

RAID-10 still has the issue that two disks failures can result in data loss.

RAID is great and RAID (except RAID-0) helps minimise the risk of data loss. There are problems that RAID can't protect against. That's why backups to another NAS or some place else are important.