@gafriedman2144 Welcome to the Rockstor community. Just a quick question, are your 8 TB drives the Seagate Archive SMR drives, or possibly other SMR drive models, if so they are not suitable for use with btrfs. Just thought I’d pop in with this one as it has come up before on the forum; just in case.
If they are please see this post in the SINGLE smr Drive Going Offline thread in this forum, and the linked post in that reference for more details.
As to your question that depends on your raid level and if you use one of the available compression options and if your data is compressible of course. There are web based btrfs calculators but the one I just found wasn’t really that friendly. I’ll have to leave this question to others for the time being I’m afraid. But that calculator should give some indication. Btrfs raid levels work a little differently to current (but traditional) raid levels though and drive groupings can be grown or have their raid levels changed on-line / in place. The general thought currently is to use either btrfs Raid1 or Raid10. Or if no redundancy is required then Raid0 to maximize the usable space.
Anyone have a link to a better btrfs space calculator?