Looking at 410.10 I conclude that the canopy of a luminaire is only suitable for luminaire wires and there connecting devices. How much branch circuit conductor is permitted in the canopy?

George I would say that if the canopy is not marked with a cu. in. capacity then none.

We are required to make all our connections in a "junction box." The fixture canopy, or base, is considered as a "box" when installed.
As such, it is required to have enough space for the usual 6" of 'free conductors." (300.14)

What is not clear is whether there is required to be enough room for a second set of wires to exit, and continue on to the next fixture. It's not even clear if there is required to be room for the switch legs.

A classic example is the "pancake" box....it has room only for wires serving the device; there is no room for these other 'associated' wires.

Personally, I consider it no longer accepted practice to mount a fixture without using a box. Apart from wiring space, the box probides a much better mounting. But I wander....:-)

Article 410.10 is as clear as mud to the issue of your post IMO. It seems to say that, but doesn't really say anything about circuit conductors. The only issue I read in it is...

"shall provide adequate space"

...I don'r read this as limiting at all to other conductors that may occupy the space of the canopy. I'd look for the word ONLY for the meaning you're implying.

Dave

I guess this boils down to the old question, if you terminate 3 #12s in a 6 cu/in pancake can you borrow .75" from the canopy?
Add a .0625" thick ring? (About the thickness of the box material)
... absurd...
If you have 5 wires in there (2H 2N and the grounds) I would have a problem in a pancake, fat canopy or not.