I think I get this one, but just to verify or clarify... (USA NEC)
When running multiple circuits (different breakers) in a single conduit, you have to derate after (is it 4?) sharing the one conduit.
But, if a single circuit (one breaker) is going to a bunch of light switches, and then the switched lines from those switches (and likely the hot line going on to other switches, LED lights do wonders for number of lights on a circuit) are sharing a conduit before they reach a junction box where they split off to the individual light strings, that's still one circuit (but more wires for the fill calculation, of course) from the "do not need to derate the wires in conduit" view?
I've seen too much rodent damage to be happy using NM cables...