[cp2141]

SAFETY!

If you go with a small number of rockets, it's unlikely you can sustain one (let alone) multiple engine failures. With several rockets arranged in a distributed geometrical fashion, you can significantly raise the probability of proper vehicle function even with several failures (as mentioned before, with the Falcon 9).

[mcguire]

Redundancy is a funny issue, though. Replacing one engine with multiple engines means you can survive one failure, but you then have a multiple of the things that can fail.