Quintuple-expansion steam engines and complexity limits

How many expansion stages remain practical in a multiple-expansion marine steam engine, and why not add still more pressure stages?

Quintuple expansion is the practical upper edge because extra stages demand very high boiler pressure and introduce flow complexity and blowback risks.

Quintuple-expansion machinery was roughly the upper practical limit reached by reciprocating marine engines. Such a plant can divide work across high-, intermediate-, and low-pressure cylinders, but it depends on sufficiently high boiler pressure at the inlet. Adding boilers or another stage between pressure groups is not a free efficiency gain. The steam path should keep moving in one direction, and extra connections create complexity and the possibility of damaging blowback. Theoretical expansion therefore has to be balanced against a reliable engine-room arrangement.