How many 12-2 Romex cables can I fit in a 2 inch PVC conduit?

The code math says about 9, and the practical answer is fewer. NM-B cable is allowed in conduit, and when you install cable in a raceway, NEC Chapter 9 Notes to Tables, Note 5 and Note 9, tell you how to count it. For a flat cable like 12-2 Romex, you use the largest dimension of the cable as the diameter of an equivalent round conductor.

The fill calculation

Typical 12-2 NM-B measures around 0.46 to 0.52 inches across the flat. Treating 0.52 inches as a circle gives an area of about 0.21 square inches per cable. Schedule 40 PVC in 2 inch trade size has about 3.29 square inches of internal area, and with more than two conductors you get 40 percent of that, roughly 1.32 square inches. Divide it out and you land at about 6 cables using the fattest measurement, or up to 9 with the slimmer product. Call it 6 to be safe and buildable.

Derating is the real limit

Six 12-2 cables means 12 current-carrying conductors in one raceway. NEC 310.15(C)(1) knocks ampacity down to 50 percent at 10 to 20 conductors. NM-B is limited to the 60 degree column final ampacity per 334.80, but derating starts from the 90 degree rating of the conductors, 30 amps for 12 AWG. At 50 percent you are at 15 amps, which no longer supports a 20 amp circuit. Four cables, 8 current-carrying conductors, gives you a 70 percent factor and 21 amps, which just clears. So if these are 20 amp circuits, 4 cables is your real ceiling, not 9.

Better approaches

If this is a conduit run to a shop or shed, skip the Romex and pull THHN/THWN singles. You can land far more circuits in the same 2 inch pipe, wet locations stop being an argument, and pulling flat cable through conduit is miserable anyway, especially around bends. If the conduit is just a sleeve for physical protection, a short sleeve is treated differently than a complete raceway run, and many inspectors will not apply fill to a sleeve, though 310.15(C) still applies if it is longer than 24 inches.