Dear Carol,
 
"... one (star fish) that had 6 legs (or arms or whatever)
Does this happen often or is it a freak?..."
 
Most of the sea stars have only 5 arms, but (not-freak) taxa having
up to 50 arms (technical name = rays) are known. Some groups of sea
stars are able to reproduce asexually by dividing its body in few
pieces. Then, each of the pieces develops and becomes a new
individual.
Accidental loss of rays and subsequent multiple regeneration, plus
all of the above, makes possible among certain groups, to "produce"
starfish with different number of rays among representatives of the
same species. For example, representatives of the family
Ophidiasteridae usually are found having 4, 5 and 6 arms (that I had
seen!). Species of this family are present in Florida. Nevertheless,
other groups of starfish have more than five rays not as a product of
accidental loss and subsequent regeneration. For example, _Luidia
senegalensis_, also present in Florida, has 9 rays. Other different
species belonging to the same family (Luidiidae) normally possess
only five.
 
Cheers,
 
  Roberto