You could create a sketch in the assembly 12 x 12 and then mate the part while nesting them inside the sketch.
There are 3rd party software for the nesting.
As Deepak indicated there is no "built in" functionality for nesting layouts.
An alternative method is to create your raw stock as a part and then create an assembly with the raw stock and all of the parts in flat pattern. You can make your raw stock transparent so as to see the individual parts. Mate the surfaces of all and then manually move the parts into position. You can permanently locate the individual parts with various mates and the "fix" mate.
While it's not really nesting....you can use the fill pattern command and chose your own sketch as the cutting sketch. You can specify gap distance on the sides, angles, etc. It will work for what you want to do.
If you upload a drawing for each of the pieces I can give it a shot. Most of the vendors I work with for water jet cutting or laser cutting ask that if you lay out the pattern to use the following criteria:
Edge of material to part spacing = .375"
Part to part spacing = .250"
Tolerance for cut out parts = +/-.010"