I guess that you are thinking for sound reduction.
Teenagers in the house? Sound system?
You will do fine with 4" frame on internal walls and use the money elsewhere on sound reduction according to your needs
Sound reduction is better achieved by using other materials along with rockwool
Instead of a 6" frame & filling up the cavity with rockwool, use a 4" frame and two 13cm sheets each side and using materials to reduce vibration in between.
If you want to cancel out low freq thumping bass it takes more effort.
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Sound passes through rockwool but it reduces the vibration.
Check out audio reduction sites, home cinema construction etc. Hotel room insul standards
Sound reduction const is according to needs, and needs differ around the house.
Then you can also think to place storage, wardrobes, book shelves on the walls affected.
I hope that helps