Also, gas dynamics in a smaller engine is simpler and easier to simulate.
And yes, it gives a lot of redundancy, if the only engine of the stage fails you are toast, if 1 out of 9 fails it's almost okay, as the most recent F-9 flight shown.
Having multiple engines means having multiple small containers that each take up metal, manufacturing, etc. All of which is dead weight as far as lift goes. By contrast a large engine is by proportion more fuel, which means you can get more stuff to orbit.
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).