How many Class A Circuits are in a single conduit isn't really the issue. The issue is separating the feed part of any Class A Circuit from the return part of the same circuit.
Keep in mind the purpose of Class A isn't so any problem with the wiring can be repaired in a timely manner; the purpose of Class A is to keep the system working before any wiring problems can be repaired - especially if something breaks once the fire hits and starts to degrade the system.
With Class A, like beads on a necklace, all of the devices are daisy chained on the circuit. The panel is where the feed part of daisy chain starts, and the panel is where the return part of the daisy chain ends. This way, both ends of the daisy chain are attached to the panel.
If the daisy chain is broken somewhere in the middle, the panel is still connected to both ends of the daisy chain. The idea is that signals can get to the panel either on the feed side of the daisy chain or the return side.
The feed and return from devices should be separate so whatever breaks one part of the circuit won't be breaking both the feed side of the daisy chain and the return side for any device.
The biggest concern is survivability of a Class A Circuit (in case a wire breaks, can the panel still communicate with the devices). Just keep the feed and return separate, and the rest can be just standard fire alarm wiring practices.
Douglas Krantz 
