I see. I tho 12 disks were "standard" as almost every storage brand (powervault, emc, equalogic) have at least 12 drives.
I might do 11z3 + one hot spare then
None of those brands are using ZFS. ZFS has a rather generalized design that doesn't forcibly marry it to some of the preconceptions in conventional technologies, which sometimes means it works differently. Conventional RAID technology has some problems "going wide" too. ZFS does offer RAIDZ3, though, which is pretty cool.
The 11 in Z3 plus one spare is a good idea, except be aware that FreeBSD doesn't actually have a way to do "hot" spares. It is more of a warm, ready-to-go spare. It gives you the optimal 8 data drive vdev width. From a failure point of view, it gives you the ability to remotely command the replacement of a failed drive while ALSO maintaining the ability to withstand up to two other failures without data loss, which is totally where I wanted to be, so I actually have a nice array in the 11-Z3-plus-spare configuration. When it can take days to move data around, there's a convenience factor to just having that redundancy.
However, that being said, RAIDZ3 is slow. Read speeds on the 11 drive array vary from 500-900MBytes/sec. Write speeds vary from 100-200. For our uses here, where this is mainly archival "nearline" style use, the speed loss is fine.
I believe someone else suggested a 2 vdev in 6+6 RAIDZ2 configuration. This would be a higher-performance solution, and doesn't include a spare drive. It would recover faster in the event you had a failed drive that needed to be rebuilt, because only the single affected vdev would need to be taking the rebuild hit, and there are fewer devices involved (and less data to analyze and fewer bits that can go wrong). On the other hand, if you do manage to lose three disks in a single vdev, your whole pool's toast.
Tradeoffs, tradeoffs.