What's the rule on conduit fill? The answer is there are many rules!
First, let's expand things a bit. Conduit is a specific kind of raceway. So, we're really talking about raceway fill--whether that raceway is conduit, EMT, NMT or some other kind of raceway. The NEC index cross-references "conduit fill" as conductor fill.
The basic NEC reference is 300.17. The NEC does not provide a specific fill number, here. It merely says the number and size of conductors can't be more than will permit heat dissipation and the ready withdrawal of conductors without damaging them.
Notice, I said the basic NEC reference. Just below 300.17, you'll find an FPN. This one happens to be highly detailed. The fill requirements are specified by first by raceway type in 342.22 - 388.22. Then, they are specified by application as follows:
Now, you can simplify all of this by understanding something the FPN doesn't tell you. Most of these various references tell you to use Table 1 of Chapter 9. It's probably left out of the FPN to avoid duplication of information (the result of which is invariably conflict and confusion), and to allow each standards committee to decide whether to refer to Table 1 or not. Article 770, for example, says that Table 1 does not apply [770.12(A)].
Going to Table 1, Chapter 9, we don't find a whole heck of a lot. One wire can fill only 53% of a raceway, and two wires can fill only 31%. And if you have more than 2 conductors in a raceway, the maximum fill is 40%.
That 40% is subject to further downward adjustment, though. The Authority Having Jurisdiction (AHJ) can require even less fill--so, use common sense. Read the FPN in Table 1 thoroughly. Don't assume that 50% fill is OK, because it's close. It's not close--it's over the limit. But don't assume 40% is always OK, either. Circumstances may dictate otherwise, and the NEC provides some examples on that point.
Some people get confused on which conductors count for raceway fill. They all count. The confusion results from misapplying 310.15(B)(6) to raceway fill. But 310.15(B)(6) is is about ampacity calculations, not raceway fill calculations. You exclude grounding or bonding conductor(s) when determining the number of current carrying conductors for purposes of selecting an ampacity table. But you don't exclude them from determining the raceway fill. Keep Articles 300 and 310 separate!
Do the math
Conduit fill requires calculation. In the past, this involved determining the circular mils for various conductors. Fortunately, the NEC now provides tables to reduce the amount of calculation in the field.
To determine how much wire you can run in a given raceway:
To determine what size raceway you need for a given wire run: