NEC - Derating Conductors in Conduit?

You are looking for the ampacity of a single conductor, not all of the conductors as a group. Number 10 THHN is listed as an ampacity of 40 amps. De rating applied of 70%, number 10 would be good for 28 amps.So all of your current carrying conductors in this raceway would have current limitation of 28 amps each.