As I was motoring around the yard today, cutting what I euphemistically call "grass", I was pondering Musk's choice to use 33 engines in that booster. It seems like he's just increasing the odds of failure by doing that. Many more moving parts and thousands more potential points of failure.
But then it occurred to me that, if he can get his Raptor 2 engines to demonstrate the same reliability as the Merlins used in his Falcon rockets, SpaceX can end up in a really good place. The Raptor 2 engines are much simpler than the Raptor 1 units, and they are cheaper to build. Simpler is generally better from a reliability standpoint.
Having 33 engines could instead provide some built-in redundancy, extra fault tolerance if you will. The booster will be able to lose an engine or four* and still accomplish the mission.
Maybe that's what drove the design choice for so many engines.
* If the booster loses one engine, it really loses two as the opposite engine will likely have to be shut down for balance. If the booster loses two engines, it then really loses four.