Why so many tubes?

For non output tubes sometimes a gain tube is followed by a cathode follower which is sort of an electronic transformer to lower the output impedance of a gain tube so it drives the next gain tube better. Goof examples are 1970s and 1980s Audio research amps and preamps.. This effectively can double the small tube count.

Another possiblity(which can be also combined with cathode followers) is push/pull voltage gain stages. This doubles the number of small tubes also. Again look at the Audio Research products from the 70s and 80s. Push/pull needn't be used for voltage gain even with push/pull output amps. And many preamps are single ended too even from manufacturers who made push/pull tube output amps.

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.