20 is the precise number of 12/2 NM cable you can fit inside a 2-1/2" Schedule 40 PVC nipple less than 24" long, without stripping the sheathing. Southwire says the width is 0.422", and Chapter 9 Table 1 notes say to treat the cable as circular of that diameter. That makes each cable 0.1399 in^2 in cross section, and the allowable 60% fill is 2.817 in^2 per Chapter 9 Table 4. Dividing gives 20.1 cables allowed.
More would fit if you strip the sheath, but then you have the issue that the NM inners are likely not lableled. Plus you'd need to provide something additional to associate the grounded conductor of any 120V circuits with the corresponding ungrounded conductor per 200.4(B). So leaving the sheath on is easier when the fill calculation complies.
Note that the 2026 NEC has revised 312.5(C) to start off "Where a cable wiring method is used". The point of this change is that a cable within a complete (non-sleeve) conduit method does not need to be secured to the cabinet, any more than individual conductors in conduit need to be. However, the panel statement indicated that this was the intent all along, even with the old word of "where cable is used." Edit: the second draft of the 2026 NEC also adds an exception to 312.5(C) to make that even more explicit.
Cheers, Wayne
