Polydactyl literally means many-fingered; this doesn't limit the mutants to six fingers. Indeed, since five fingers is the 'norm' (the
pentadactyl limb concept) for
vertebrates any
more than this is considered 'polydactylous' - six, seven, eight and so on. However, this is somewhat confusing if you consider
horses and
whales (both vertebrates) which don't seem to fit the pattern. The standard explanation is
embryological - the
foetus has all five fingers, but they are lost or fused together during
development.
However in an essay called "Eight Little Piggies" (the title essay of a compilation), Stephen Jay Gould makes the case for the pentadactyl limb as a 'frozen accident'. In other words, there is nothing especially unusual about limbs that have more or less than five digits. Some evolutionary experimentation with other numbers may have taken place. This makes sense if you consider the potential for extra fingers that polydactylous mutations show. Also, frogs have four fingers on their front feet - although the panda's apparent sixth digit (the false thumb) is not a true digit.