12/2 NM cable is considered a round conductor with the major diameter of the cable, the width, used as the diameter of a round cable when figuring conduit fill. Depending on the size of the conduit, 1/2" EMT would allow a single 12/2 NM cable. 3/4" EMT would allow maybe 2 without actually doing the math. Put a box at the end of the conduit and use individual THHN conductors in the conduit to get more circuits. Then it is the normal rules for conduit fill and multiple current carrying conductors. If you want the exact detail please provide the conduit size and confirmation that you are speaking of 12/2 w/G NM cable.