Several populations of cats regularly produce individuals with polydactyly, a.k.a. extra toes (the usual number for cats being five toes on each front foot and four on the back). Polydactyl cats have between 1 and three extra digits on any given paw. This phenotype was recognized and discussed by Charles Darwin as an inherited trait.
Many breeders avoid mating two polydactyl cats, but those who have done so report no problems. Polydactyly is considered a harmless mutation and so something a responsible breeder can selected and breed to promote if thy like the look of it. Historically cats with polydactyly have been referred to as mitten, boxing or thumbcats.
Hemingway house is home to a population cats about half of which have an extra toe on their front paws. They are descended from a polydactyl cats called Snowball who was given to Earnest Hemingway. Most have six toes on their front paws.
The only currently recognized breed that allow extra toes is the Pixie Bob.This breed is reputed to descend from barn cat x bobcat matings but such a combination is biologically unlikely and not objectively confirmed. A pixie bob may have 6 or seven front toes.
American Polydactyl Cat
The American Polydactyl Cat is breed specifically for the extra toe trait with the goal of developing a recognized breed.
Munchkins are cat with short front legs, they also sometimes show polydactyly.
Extra toes have also been documented in a number of breeds in which is is considered a deformity to be avoided, including Bengal. It is also not particularly uncommon in mixed breed cats.