Fast Cache amount of disks

Hi guys

Based on my experience with VNX back in the old days I know that I have to have 3 SSD Fast Cache disks to make it work otherwise the write cache for Fast cache has been disabled (2x Disk for RAID1 and one Hotspare of the same Fast Cache disk type). If this is not corrent please correct me

Is this the same for Unity Fast Cache or can I build up Fast Cache with only 2 Disks resp. a multiple of 2? In the best practice guide https://www.emc.com/collateral/white-papers/h15093-dell-emc-unity-best-practices-guide.pdf there is not statement about this.

Cheers and thanks