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.