More tubes are required if it's a line stage + phono preamp combo. That is the only exception. I have seen basic preamps with a single tube that work exceptionally well. A single tube, set up correctly has considerable gain. Most amplifiers have extremely sensitive, high impedance inputs. You may not know this, but preamps (line stage) attenuate the signal more often than amplify anything! An incoming signal of 0db to most amps would cause considerable damage to your amp, speakers AND possibly you in most cases. This is why passive preamps (with zero active components) work so well. So- if zero tubes/transistors works so well in the line stage, why should a preamp have multiple tubes? If there are tone circuits, etc.. extra gain stages are necessary. A basic preamp does NOT require a half dozen tubes give or take a couple.
To answer your question directly- If a design can work excetionally well with, lets say 5 components, why use 10? The individual who chooses to make a 10 component design isn't as good a designer. Complexity does not in any way prove an engineer is good. Designs that are complex when they don't have to be shows very poor judgement. If there are two machines and one has more parts in it than the other, yet they both work exactly the same, wouldn't you choose the machine with fewer parts? It's likely more thought went in behind the model with fewer components and it will work better anyway. As a last comment, devices with fewer compoents statistically break down less often.
To answer your question directly- If a design can work excetionally well with, lets say 5 components, why use 10? The individual who chooses to make a 10 component design isn't as good a designer. Complexity does not in any way prove an engineer is good. Designs that are complex when they don't have to be shows very poor judgement. If there are two machines and one has more parts in it than the other, yet they both work exactly the same, wouldn't you choose the machine with fewer parts? It's likely more thought went in behind the model with fewer components and it will work better anyway. As a last comment, devices with fewer compoents statistically break down less often.