RAID-Z
The strongest valid recommendation based on exact fitting of blocks into stripes is the following: If you are using RAID-Z with 512-byte sector devices with recordsize=4K or 8K and compression=off (but you probably want compression=lz4): use at least 5 disks with RAIDZ1; use at least 6 disks with RAIDZ2; and use at least 11 disks with RAIDZ3.
To summarize: Use RAID-Z. Not too wide. Enable compression.
RAIDZ Configuration Requirements and Recommendations
A RAIDZ configuration with N disks of size X with P parity disks can hold approximately (N-P)*X bytes and can withstand P device(s) failing before data integrity is compromised.
Start a single-parity RAIDZ (raidz) configuration at 3 disks (2+1) Start a double-parity RAIDZ (raidz2) configuration at 6 disks (4+2) Start a triple-parity RAIDZ (raidz3) configuration at 9 disks (6+3) (N+P) with P = 1 (raidz), 2 (raidz2), or 3 (raidz3) and N equals 2, 4, or 6
The recommended number of disks per group is between 3 and 9. If you have more disks, use multiple groups.