I don't understand AFCIs well enough to know for sure. Obviously a GFCI would work and do its job no matter how many circuits...
... except of course that it could detect a leak from hot to neutral anywhere among the circuits. Whereas a GFCI on a single circuit will detect a leak to/from another circuit.
Now what I've seen of the AFCI arc signature recognition, they show the wave pattern of a circuit with nothing on it but the arc fault. But of course AFCIs are supposed to be able to detect arcs on a circuit with many things plugged into it. So it makes sense to me that they should be able to detect on any circuit in the entire subpanel fed from it. Even if a circuit is loaded with lot of noisy devices, I think that the arc's intermittent current flow should stand out well above the noise, meaning that you aren't losing sensitivity.
The only real drawback is that an arc on any branch from the subpanel shuts down the whole subpanel.
