Steve:
Based on the NEC, yes one device (as Ryan stated) counts as two conductors, as you calculated.
The 'common sense' rules that I/we use allow for a little extra room for 'big' devices (GFI, dimmer, etc) Heck, a little extra room never hurts. 22/23 CI plastic new work, 1 gangs are the only 1-gangs we use. Comm is 4x4 deep, or need be a 3-1/2 device, gangable (gem).
With my AHJ hat on, you may be surprized how many 'bof-fill' red stickers are written.
John
