12 standard size hens in 48 square feet, yes.
Bantams usually do OK with only 2 square feet each.
So 8 bantams require 16 square feet
And if you have 6 big chickens at 4 square feet each, that's 24 square feet for them
16 + 24 = 40 square feet needed for your chickens, and the coop has 48...
Using one set of "nestboxes" to hold food & water is a good idea, because you certainly don't need 6 for the number of birds that can live in that space.
If the silkies are bantam sized, then I think the coop is large enough for 3, maybe 4, but not more than that. (Or 3 to maybe 4 of any other...