[guyhood]

I am installing numerous two gang receptacle boxes in a shop/barn area.
The boxes are regular metal 4 square outlet boxes connected to 1/2" EMT conduit. There will be six #12awg wires coming in to each box and two duplex receptacles in each box. I will be putting wire nuts on the pigtails, so there will three red wire nuts also. I am not sure of the formula for how many wires and devices you can put in a regular 4x4 box. Does it have to be a deeper bigger box for this many wires and devices, or am I okay.
Thank you, Guy

[tkrussell]

The #12 wire each need 2.25 cubic inch, each device counts as two wires, so there is a total of 22.5 CI needed. I am assuming there is no ground wire. If there is a ground wire all will equal one wire.

The box needs to be no smaller than a 4"x4"x 2-1/8" deep box, not considering the cover. If this box is surface mounted using what is called a raised cover to hold the devices, it also has volume of space which is stamped on the inside, and can be added to the outlet box space.

[Stratmando]

In addition, if wire goes in and out of box, it counts as 1 wire, if you cut the wire, it now counts as 2 wires.
Don't skimp on box size. You may need to add more later. Dimmers and GFI's are large devices. Stranded wire can be less stressfull(physically) on devices, than solid.