Why so many tubes?

if the op is seriously asking the question, the answer is quite different from preamps and power amps

each has reasons, different reasons, to use few vs multiple tubes 

I suspect a troll or lazy OP. All it takes is to Google “audio vacuum tubes: advantages, disadvantages, and uses in audio”. Also, learning how an audio signal is amplified is also readily available online.

@czarivy

The speed of your car is not dependent on its cylinder count.

Not a troll thread at all. Seems like a complicated way to reproduce music. And yes a little bit of laziness involved, but electrical engineers make lousy writers as the subject is a difficult exposition. 
So some of tubes are the same as the power transformers? And I am open to SS to being more complicated. 

The speed of your car is not dependent on its cylinder count.

But more importantly there is a general relationship between the number of the cylinders and the output horsepower.  It's actually a good analogy.