ECN Forum Forums Code Related Discussion NEC & other Code issues how many wires in a box

8 #12s would be max. with a blank cover.
You must deduct 2 for the receptacle, 1 for all(not each) of the grounds, and on metal boxes with internal clamps, 1 for all the clamps. This leaves 5, (or 2 cables).
#12 requires 2.25 cu. in. per conductor. If you know the volume, do it that way.