Q. I notice the minimum number of disks required for a mirrored Storage Space is two, however for most other types of resiliency additional disks are required. Why only two for mirroring?
A. The two-disk requirement is actually a special case to enable the use of a two-way mirror in client scenarios, since Storage Spaces is also part of Windows client. If using a two-way mirror in a cluster scenario you should have at least three disks.
This TechNet article has a break down of the minimum number of disks required for the various types of Storage Space configuration. For example, two-way mirror only requires a minimum of two disks, while a three-way mirror requires a minimum of five disks which helps ensure quorum in the event of some break in disk communication.