Normaly I'd say two--rule of thumb is one box per four birds, but at least two boxes. However since you have silkies who are known to be hard-core broody hens I would probably put three boxes in so you have one available (hopefully) when the broody bug bites them.
I can tell you that it won't matter how many nest boxes you put in, they have their favorites and I have found up to 3 of my silkies trying to squeeze into one nest box, even though I have 7 others empty. ahhhh, silkies.