Thank you, that is brilliant!
If I understood correctly, "airlines" are binded by p.121 to keep certain standarts (I am hugely unfamiliar with FAA regulatory system) and the document you referenced gives guidance to inspectors what are the type specific limits.
Our 321 are operated 212Y and there are 3 spare jumpseats available to passengers under certain restrictions. On top of that, infant number is limited to 10 by available emergency equipment (lifejackets ...) which brings total head count up to 225 theoretically. I am led to believe this would be illegal (not that it ever happened), but I found no reference anywhere. I think the limit may come from FAR or BCAR aircraft certification requirements, yet I need to estabilish "paper" reference to clarify the topic.
FD
(the un-real)