Hi Carol,
 
Six arms on a starfish is uncommon, but not extremely rare.  I'd estimate that
you could probably find a "six" among every few hundred "fives".  This is
usually the result of an injury (loss of an arm), followed by regeneration of
two new ones.  Occasionally you may find a "four", or a specimen with more
than six, but I would consider those rare.  I have also found an occasional
one with a forked (Y-shaped) arm.
 
Paul M.