With "fill" you are going to add up fast. You use the dimension of the cable as it's widest point as a diameter, then compute based on a circle that size. A 12/2 Romex "fills" about the same as a #1 or #2 THHN (9-13 in a 2" PVC) The other problem is derating. Once you get over 4 Romex in that pipe you will start eating up your ampacity, beyond the hit from 240.4(D)
