I have 7 silkies who hatched 5 days ago
They're mother is a black sultan and their father is a brown sultan
Are they Silkies or Sultans? Since those are two different breeds, I think one must be a typo.
They're mother is a black sultan and their father is a brown sultan and both of them are purebred with 5 toes on each foot. And by the way the chicks with the 5 toes on each foot are: one of the brown ones and one of the white one and the white one with black lines on its back. So I'm not sure if the others are like purebred
"Purebred" usually means both parents are the same breed, and all grandparents are the same breed, and so on for enough generations that they will mostly produce chicks of the same kind as themselves.
So either they are all purebred (both parents and all chicks), or they are not. You cannot get some chicks that are purebred and some that are not if they came from the very same parents.
Probably they are all purebred, but have a few not-quite-right genes. That is fairly common with purebred animals.
I had asked about whether different colored chicks had different numbers of toes because I thought they might have different parents. Since they all have the same parents, it makes sense that you are seeing almost all possible combination of colors & toes.
As regards specifically the toes:
All 7 chicks have the same parents.
Both parents have 5 toes per foot, but more than half of the chicks have 4 toes.
If each parent has the gene for 4 toes and the gene for 5 toes, I would expect about 1/4 of the chicks to have 4 toes. Obviously, you have a much higher rate than that, which could be because there are only a few chicks total, or could mean that something else is causing the 4 vs. 5 toe difference.
I have read that chicks with the genes for 5 toes will sometimes have 4 toes anyway, possibly due to incubation conditions or possibly based on some other genes that are not fully understood. So that may also be causing some of the chicks to have 4 toes.