Any multiple of two that is greater than two, up to the number of available ports. RAID 10 is a mirror (two times) of a RAID 0 (two or more drives). The bare minimum is four, two RAID0 of two each.
Technically, you could mirror more than two times. How about 9 drives in three RAID0 of three drives, with three copies? It would be a little silly, but you could do it.