It depends on how many eggs you want incubated. You need to be sure that the Silkies are able to cover all the eggs underneath them, and since they are bantams, I would limit the number to about 8-10 eggs per hen (they will sit on larger numbers if you allow it but you will get a better hatch...
I've had dozens of breeds over that past 50 years, and Silkies are the best brooders and mothers that I've ever seen. We had Black Silkies and they raised a number of broods for us.