Conduit, wires, breakers.

Conduit size does not matter when it comes to de-rating. First you must calculate, based on circuit amperage size, what size wire you need. You can have three current carrying conductors in a conduit before you nee to de-rate. When you de-rate for 14,12, and 10 gauge you are allowed to use a higher starting current for your calculations. Example, #12 wire you can use 25 amps for your calculations. ( although you can never put #12 wire on larger than a 20A breaker) When you have up to and including 6 #12 current carrying conductors, you have to de-rate 80%. 80% of 25 amps is 20 amps, so, you can have up to 6 #12 current carrying conductors in a raceway before you have to up-size the wiring accordingly. Once you determine the amount of conductors in the conduit, and, whether they need to be up-sized and to which size, then you base conduit size on conductors contained therein. Whether or not a conductor is considered a current carrying conductor is a whole new topic for debate, but, nowadays with almost everything being electronic, it is wise to consider both ungrounded and grounded conductors as current carrying.

You have no idea what you are talking about. When determining how many wires can fill a conduit, conduit size absolutely matters, as well as de-rating value.