The documentation in http://rockstor.com/docs/data_loss.html#data-loss-prevention-and-recovery-in-raid1-pools starts like this:
Blockquote A raid1 pool in Rockstor consists of exactly two drives. A copy of data on one drive exists on the other one. So a raid1 pool is simply a two drive mirror.
A btrfs raid1 pool can have more than two drives and such configurations are supported by the Rockstor UI. I think it is important to know that it is possible to aggregate multiple drives with resilience using raid1, especially as RAID5/6 are not considered ready for production use.