WHY IT IS shows 8.83TB?

I have ex4 with 12tb but it is show in home menu 8.83tb why is that?

I used raid 5 during installation process

Because you used raid 5.  Raid 5 uses redundant parity data that is distributed across the disks and this parity data uses some of the space.  This allows the raid to ‘reconstruct’ a failed drive on the fly and continue to function. When the failed drive is replaced, this parity data from the other drives is used to replace the data that was contained on the failed drive.

Thanks a lot for your support and information as well