Question about Synology Rs816

Of course you can install only 2 disks. However, I would think twice about whether you want a RAID after all. Because if you define a volume that extends over several disks, your probability of failure increases with the number of disks, i.e. with 2 disks twice as high, with 3 three times as high, and so on.

Yes you can. I have only installed 2 so far