A.Master electrician Rex Cauldwell responds: For practical purposes, the maximum number of 12-gauge THHN conductors for a 3/4-inch EMT conduit is nine.

The answer would be different for another type of conduit (for example, ENT), or if the conductor had a different insulation type or different outside diameter. Although the NEC allows up to 16 current-carrying THHN conductors in such a conduit, it also requires that for any number of conductors over three, you have to derate the capacity of the conductor. The derating isn’t significant until the number of conductors exceeds nine. For 10 to 20 conductors, 12-gauge wire is derated down to 15 amps.