You can put 9 THHN, 14 or 12 AWG current carrying conductors in a raceway before derating becomes an issue.
310.16 shows 14 AWG THHN rated 25 amps
Table 310.15(B)(2)(a) shows a 70% downward adjustment for 7 to 9 current carrying conductors
25 amps x .7 = 17.5 amps
The 14 AWG is still OK for a 15 amp breaker.
For 12 AWG
310.16 shows 12 AWG THHN rated 30 amps
30 amps x .7 = 21 amps.
The 12 AWG is still OK for a 20 amp breaker.
There are areas that must use EMT for residential.
For derating purposes you can use the 90 C column of 310.16 if you use THHN.
