How common is it for a five-armed starfish to have six arms?

"The starfish does have a remarkable ability to regenerate lost arms, sometimes growing two where there was one, meaning that sixarmed creatures are not unheard of."

(In a comment on that article: "Growing up on the west coast of Norway, I've seen lots and lots of starfish. this species as well. I've found them with everything from three to 16 arms. This species I've found with variations of four to 6 arms."

http://www.dailymail.co.uk/sciencetech/article-1222404/Starfish-record-eights-legs-British-coast.html#ixzz3kBIpcorT

"Most starfish have five arms that radiate from a central disc, but the number varies with the group. Luidia ciliaris has seven arms, members of the Solasteridae have ten to fifteen while the Antarctic Labidiaster annulatus can have up to fifty. It is not unusual in species that typically have five arms for some individuals to possess six or more through abnormal development."

https://en.wikipedia.org/wiki/Starfish#Anatomy

A five armed starfish will never have six arms, because a starfish with six arms is a six armed starfish.