depends on your trade-off between performance and space. the more disks in a raid-group the better performance
it is easy with the SAS disks:
1 raid-group 20+2+2
it's a bit different with the SATA disks because a SATA raid-group is limited to 20 disks
so:
18+2+4 spares
you can also have 2 raid-groups but it doesn't make sense as the performance is worse and you are getting the same capacity (18 data disks)
(9+2) + (9+2) + 2 spares
(this is all assuming the use of RAID-DP)
