Several options:
Four data disks per controller: pro: utilize CPU/RAM resources of both controllers, cons: usable space per controller. No hot spares.
C1: 6 disks (4D + 2P) (no hotspare)
C2: 6 disks (4D + 2P) (no hotspare)
Eight data disks on controller 1: pro: maximize space on 1 controller, use 2nd only for cluster failover. cons: CPU/RAM resources of 1 controller. Other isn't doing anything. No hot spares
C1: 10 disks (8D + 2P) 0 HS
C2: 2 disks (RAID4: 1D + 1P) 0 HS
Seven data disks. Same as previous, but with an hot spare for controller 1
C1: 10 disks (7D + 2P) 1 HS
C2: 2 disks (RAID4: 1D + 1P) 0 HS
There are probably other options. All depends on your needs.
What you've certainly noticed is, that if you're willing to work without spares (but still use RAID-DP), you end up with 8 data disks.
Mind you some of these scenario's go against NetApp best practises, but in this case, FAS2020A with 12 disks you gotta be creative.
Regards,
Niek