Well, for any hatch you can probably expect ~80% fertility (You should be able to expect 100% if the breeder and their chickens are all doing their jobs properly, but things happen). When shipped, that number more than likely goes down by another 20-40% depending on a variety of variables- treatment during shipping, temp during shipping, how long ago the eggs were laid vs. time taken shipping (the longer the time between laying and incubating, the less the hatch rate is), how long you let the eggs settle, incubation fluxes in temp and humidity.... Sometimes the stars align and if you order 6 eggs, you get all 6 to hatch. Sometimes the opposite stars align and nothing hatches at all.
I know this is completely unhelpful in determining what to do, so in the end I would say order 6 eggs and see what you get; at worst you have to order some more, at best you get the number of silkies you want and don't have to order again.