Common, yes.
Normal, No.
People need to be conscious of what they are hatching. They should NOT be hatching from 4 or 6 toed Silkies... or they should NOT try to claim quality.
It is hard work to get and keep only good stock. Out of more than 100 chicks I hatched from parents with correct toes... Only 1 ended up short a toe.
If all the parent stock has correct toes and you are seeing a lot of incorrect at hatch... then one or both parents are throwing a hidden fault. Any breeder should be able to see the toes and disclose what you are getting instead of letting you discover it and wonder. Of course, I do realize things can get hectic and be easy to miss when you have a bunch of chicks.
The whole reason most people buy from a breeder is for better quality than you get at the hatchery... sadly it just isn't always true. I personally make the request for people who buy my birds to please let me know if they catch something I didn't, because I cannot fix or change what I don't know.
Most average people won't have the heart to keep the bad genetics out of the gene pool. It's up to responsible breeders to make sure the poor genetics don't make it out in the first place, IMHO.

It IS a very HARD line to toe!