Echinaster luzonicus, commonly called the burgundy (or maybe red) Linckia commonly has 6 arms. It is not uncommon in many species of seastars. Sometimes it is an abnormality, sometimes it is characteristic of the species. Its basically just not true that seastars always have 5 arms.
Only some seastars will reproduce via arm drops. Many Linckia can do this under appropriate conditions. I never recommend people cut off the arms of Linckia to do it as most will simply kill their star. Any arm may break off, and in extreme cases all arms of the star can "walk" off in opposite directions pulling the star apart, with each arm then regrowing another star. How this happens is not well understood.
