Since most manufacturers of single station smoke alarms are limiting you to between 10 and 12 units on a circuit, what does one do when you need more than 12 units for a home?

I suppose you will need to find a manufacturer that doesn't have the restriction or find one with a "repeater" type adapter to link 2 loops.
In real life I would expect a home that big to have a real alarm system. Even my wife's micro mansions (2000-4000 sq/ft) had a system that picked up the smokes, CO, pool alarm and intrusion alarm.

Greg some areas will not allow a fire alarm system to take the place of the required smoke alarms.

I don't know the solution to the question.

Bob that seems strange to me but I never underestimate the absurdity of government.

Off the cuff, I don't see a problem.

The limit seems to be based upon the current they need when in alarm. This has nothing to do with interconnection.

I would think that you could continue to link them all together with that third wire, even if the detectors are powered by different circuits.

I just inspected a home that had a need for 31 yes THIRTY ONE, smoke alarms and while it appears that it would be possible to interlock them electrically with some sort of relay system- the manufacturers tell me that that's not allowed due to the listing. As for a Fire alarm system- The State of Michigan will recognize it as satifying the requirements as called out in the Michigan Residential Code (same as the IRC only with the Michigan spin).

I am wrong about the alarm system. My wife says no, they use stand alone smokes.
Everything else is on the central system.

Give me a break, I have already been punished

Reno, what exactly does go down that 3d wire and would the smokes being on opposite phases change anything?
Do all brands work the same way?
I have never really looked into this.

As I understand it, that third wire only links some contacts together ... it there's continuity in one detector, they all sound off.

Yes that true John but if you have third wire interconneted and also the netural line as well as common return [ i have little trouble to grasp this idea how it work ]

but for hot's line it can be on common or diffrent phase or leg but i am sure but not excat answer for this one but i am thinking out loud that both red [ interconnection ] and white wires to actavie it

Merci , Marc