One goal of BFR is Mars, from which you can only get back if you can produce fuel on the ground. Which kinda rules out RP-1 as propellant. Hence the need for a methane engine.

Unrelated to the number of engines here, obviously; just another point to consider. And if they end up designing a new engine they could just as well apply what they have learned in the meantime. Merlin is a very conservative design, favouring simplicity and cost over thrust. If you plan for re-using your rocket a thousand times, cost of an engine isn't that much of an issue anymore and you can prioritise other aspects.

> from which you can only get back if you can produce fuel on the ground

Is this actually realistic?

An important source of loss in smaller engines is viscosity. Proportionally, more of the working fluid is in contact with the internal surfaces, more of it is inside the boundary layer, which is the only place where viscosity matters.

For a large engine, your boundary layer goes up as the area (square), and the contained goes up as the volume (cube). (The growth of the boundary layer goes up as something like the square root.) It's one of the few places in rocket science where the square-cube law helps provide a useful result, despite the insistence of many a misguided layperson. Even here, it may be something like a 2.5-cube law.

That's interesting. This scaling law is a weird thing (I vaguely recall a myth about Galileo considering it as the most mystifying law of nature or something like that).

However, when you scale an engine down, the surface areas decrease less than the volumes. That means that you have more available area for your "pipes", and thus you can make them relatively larger. That allows lower fluid velocity for the same flow rate.

Thus assuming viscosity is an issue because it prevents fluids to flow as quickly as the cycle requires them too, making pipes larger should improves things, shouldn't it?

They certainly could use more smaller engines for the BFR, but just because one dimension of the design benefits from smaller engines doesn't mean the optimal design uses a nano-scale rocket array.