If you are using single redundancy, the effective net storage would be approximately 2790GB x 5 disks (the sixth is used for redundancy) which is approx 13,950GB which would probably show up as 13TB on the front panel. If you are using dual redundancy, the effective net storage would be approximately 2,790GB x 4 disks (the other 2 used for redundancy) which is approximately 11,160GB which would show up on the front panel as 11TB. I would wait until the unit is completely finished synching before checking the available volume.
Remember that no single storage unit is to ever be used without a backup of your data. Murphy is always lurking.
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad