The 12 alarm, 18 enunciator limit seems to be from NFPA72 not the hardware limitation of the unit. They seem to be saying the unit itself will run 24 but NFPA is limiting them.
It still seems to be a single station smoke alarm, with optional interconnected devices, not a centrally controlled alarm system.
I suppose the final answer will be how it is listed.
For the original question, it does seem that NFPA limits you to 12 alarms in a house if they are all going to be interconnected. I suppose you need to take your question to them if you need more than 12. I am sure they have an answer