Looking to explore the inverse point of this question:
As stated in the answer to the above question, fewer, bigger engines reduces parts count and increases reliability at the expense of thrust range (relevant for reusable boosters such as Falcon 9), but what happens when you want to go really big?
The largest single combustion chamber liquid-fueled engine ever successfully flown was Saturn V's F1, last flown 45 years ago. It has no equal today (not considering solid-fuel boosters such as the Shuttle SRBs); even the Space Shuttle main engines develop only about half as much thrust.
The Falcon 9 demonstrates that nine engines are enough to provide the variable range of thrust necessary to land an expended booster. So what trade-offs lead SpaceX to employ large numbers of engines for their proposed big heavy boosters (one proposal called for 42 engines, another calls for 31) rather than develop bigger engines to use in smaller numbers?
I know that one problem faced during development of the F1 was combustion instability. Is this the dominant factor inhibiting the development of bigger engines, or are there others, and if so, what? The modern "benchmark" seems to be the SSME; what is impeding development of anything substantially larger, like, for example, that of the Sea Dragon (again, limiting the question to liquid-fueled engines)?