I'm not quite sure what fredrick was saying...??
First of all, sills and headers have to do with window and door framing....
You're referring to the top and bottom (sole) plate.
A typical staggered stud wall configuration would be 2x6 plates, with 2x4 studs staggered every 12". This will put your studs at 24" on center (the space between the studs from the center of each). Then you would weave the rock wool, or whatever insulation you're using, through the center of the studs.
Of course, you could increase the size of the plates ( 2x8, 2x10) as square footage and budget permit.