I would not get one or two if they are going into their own coop. They are social animals. If you get two and lose one it will not be good.
You might want to follow the link in my signature before you decide on how big a coop you need. It's not just a square feet per chicken thing, there are a lot of variables. In general the tighter you squeeze them the more likely you will have to deal with behavioral problems, the less flexibility you have to deal with issues, and the harder you have to work. Bigger is better.
Where are you getting the Silkies? Not all are bantams but most are. Where you get the Silkies from should be able to tell you their size.
A lot of people keep Silkies with their regular flock and do OK. Integrating any chicken can cause problems even if they are the same size and breed. Different ages, size, or general appearance can make it more challenging. If you can tell us the size of your coops/runs in feet (photos can help) how many and what kind of chickens you have, and how you manage them (free range during the day, when do you lock them up at night and more importantly let then loose in the morning, the age of the Silkies, general info like that the more we may be able to help you. Another issue with Silkies is that they cannot fly. You may have to make special arrangements for hem to roost of get to a nest.
While it may be possible to put Silkies in with your regular flock the right answer for you might be to keep them separated, at least at night. If they have a lot of space during the day thy might be able to range together.
