For a NAS, I would set it up like this:
2 disks: RAID1
4 disks: RAID5
More than 4 disks: RAID6
So if you wanted all the disks in one massive array, you should do RAID 6. But you could also split it up into 2x 4disk RAID 5s or some other combination. It all really depends on what you want, as there are tradeoffs with any setup.
With one massive RAID6 array, you'll have 36 TB with a tolerance for 2 disk failures. The downside is that writes are slower, but I don't think it'll be noticeable if you're just using it for file storage.