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You will need to use larger outlet boxes to accommodate the larger and
additional conductors especially if you will be going from box to box. 4
11/16" x 2 1/8" square boxes will work best. Check tables 314.16(A) and
(B).
The number ten wires will be difficult to attach to wiring devices.

I'm planning on either using two two-gang boxes, or a single four-gang
box. I'm only entering 2 10's, and 6 12's, I think that just about
any two-gang has enough cubic inches to qualify for those.



*A deduction of two conductors must be made for each wiring device based on
the largest conductor. A deduction of one conductor is made for clamps
inside the box. 2.25 cubic inches is required for each #12. 2.5 cubic
inches is required for each #10.

4 x 2.5 = 10 cubic inches for the devices.
2 x 2.5 = 5 cubic inches for the two #10's.
6 x 2.25 = 13.5 cubic inches for the six #12's
1 x 2.5 = 2.5 cubic inches for the clamp.
Total = 31 cubic inches.

If you use 3.5" deep gem boxes ganged together you should be fine. Don't
skimp on cubic inches to save a buck. It will be a pain pushing all those
wires into the box with GFI's attached and #10 wire.