Sure you can! Is she fully broody, and secluded from other layers? (Mine hoarded eggs, and one day, she had 24 under there!. She was against a cold brick wall, so I put them in the bator, got 15, which I thought was OK!
A Silkie mama hatched 14 out of 18. They can flatten out and cover them all..it's really quite amazing!!
I currently have 21 eggs under a Standard Cochin who decided to go broody on one egg..lol.
The general rule is they can cover about a dozen eggs the same size they lay. So, a dozen Silkie eggs for a Silkie is perfect. I usually give my Silkies about 6 standard eggs or 12 banty eggs. I don't usually push the limits over 12 because I worry about her covering them all, especially while the weather is still cool.