Sorry if this comes too late...
NL-SAS drive are generally treated as SATA drives, and aggregate raidsize limit is set to 20 for RAID-DP.
Of course, a raidsize limit of 20 is very annoying for a 24 disk shelf.
Depending on your workload and performance requirements, you might want to set the hidden raid.raid_dp.raidsize.override option to true.
This will push the raidsize limit for those drives up to 28, and allow you to create a 22 or 23 disk aggregate with a single raid group.
Also, note that NetApp "4TB" disks have a real capacity of 3718 GiB (3807816 MiB).
If you go for a 23 disk RAID-DP aggregate, that's still 68.6 TiB of usable space.