There will be 5 2tb drives total on a raid 6 with 6tb total space. I can then go on to add up to 6 more tb for a total of 12tb max storage space. Remember that the double redundancy of raid 6 XOR extinctions = c(n-2) vs c(n-1) [n-1 is raid 5] where n is the number of drives and c is the capacity per drive.

So 8 2TB drives has a total of 16TB minus the space of 2 drives or 2TB each gives me 12TB max storage in the future if I was to add 3 more drives.